BOB GARFIELD:  Meanwhile, in coverage of the primaries, physics is at work.

FEMALE CORRESPONDENT:  Senator Bernie Sanders has new momentum

After a big weekend in the Democratic race.

MALE CORRESPONDENT:  Gaining momentum, a Saturday sweep for Senator Bernie Sanders.

FEMALE CORRESPONDENT:  Senator Bernie Sanders gained some momentum following a decisive sweep over the weekend.

BOB GARFIELD:  His wins last week in Alaska, Hawaii and Washington are expected to propel him to more victories because that’s how momentum works. Or at least, that's what the political press tells you. But we've been here before, in the early days of March, for example.

FEMALE CORRESPONDENT:  The Sanders camp is pleased with their  momentum after winning Michigan this week. That’s a huge upset for Hillary Clinton.

BOB GARFIELD:  And then a week later:

FEMALE CORRESPONDENT:  Clinton’s lead has helped put a stop to Vermont Senator Bernie Sanders’ momentum.

MALE CORRESPONDENT:  The Democratic frontrunner stormed through Super Tuesday and threw a big bucket of cold water on Bernie Sanders’ momentum.

MALE CORRESPONDENT:  And they almost all, probably all, of all the momentum Bernie Sanders had…

BOB GARFIELD:  At least, Bernie’s poll numbers have been steadily increasing. We've been hearing dubious momentum claims about the rest of the gang for months.

MALE CORRESPONDENT:  Last night, Marco-mentum ran smack into Chris Christie.

MALE CORRESPONDENT:  In Ohio, Trump failed to defeat the popular Governor John Kasich. The win gives Kasich momentum.

MALE CORRESPONDENT:  Meanwhile, former Florida Governor Jeb Bush surging in a new poll. He’s really picked up momentum. And Donald Trump is sinking.

MARK LEIBOVICH:  The reason journalists like it is that it’s something you can completely make up.

BOB GARFIELD:  Mark Leibovich is the chief national correspondent for the New York Times Magazine, a man who has made a particular specialty of identifying political and media buzzwords and beating them into smithereens.

MARK LEIBOVICH:  Basically, like so much else in political dialogue, it's utter nonsense and yet we play along because that's what we do. I mean, it was somewhat clever when, I guess,  George Herbert Walker Bush used to talk about Big Mo because I think he was adopting a sports cliché.

BOB GARFIELD:  Hold it, let's hear that.

  [CLIP]:

FORMER PRES. GEORGE W. BUSH:  What we’ll have, you see is momentum. We will have forward “Big Mo” on our side, as they say in athletics.

  [END CLIP]

MARK LEIBOVICH:  Big Mo? I guess the only marginally clever  adaptation was Joe Lieberman, in his campaign for president in 2004, did a number of plays on Joementum, which, you get it, it kind of rhymes, which, you know, in retrospect was probably the high point of Joe Lieberman’s presidential campaign.

BOB GARFIELD:  Is there anyone else you can think of who  actually rode momentum into elective office?

MARK LEIBOVICH:  I mean, I think Donald Trump, if you look at the earlier part of this campaign, was constantly citing polls. And this was at a time when, you know, politicians didn't typically do that. That was one of the many norms that he violated. I think what that did was it was a created a sense that he was, in fact, winning. I mean, his whole selling point was, I am winning, I will win, America will win. I mean, momentum is real insomuch as success is real, and success often continues. But claiming momentum when it doesn't really exist, I don't think it has very much utility.

BOB GARFIELD:  What language do you turn to, to describe reality without assigning it some sort of a Newtonian property of the universe?

MARK LEIBOVICH:  I think the first Newtonian property of the journalistic universe is that reporters should refrain, really from here on in, from trying to place a name in any kind of momentum construction. Look, it's very, very hard to predict a presidential race, especially this presidential race. And one of the many empty and sort of silly indicators people look to are the ephemeral things like momentum that can’t ultimately be measured. Even if they’re proven wrong in the next election, no one’s going to remember anyway. We should try to be operating in the realm of what is quantifiable, what is real. And quite often, you know, momentum is not real. It’s just a line.

BOB GARFIELD:  Mark, thanks so much. [LAUGHS]

MARK LEIBOVICH:  [LAUGHS] Thank you, Bob.

BOB GARFIELD:  Mark Leibovich is the chief national correspondent for the New York Times Magazine.

It's Newton's second law that’s about momentum, how it takes a larger prolonged force to get an object up to speed, and also to bring it to a halt. We heard Leibovich say that journalists covering politics don't really do the math. But John Sides does.  He’s co-author, with Lynn Vavreck, of The Gamble:  Choice and Chance in the 2012 Presidential Election. John, welcome to the show.

JOHN SIDES:  Thank you very much for having me.

BOB GARFIELD:  You know, I was looking at a Nate Silver story from 538.com, and this was from 2010, and he was discussing momentum and proved, I think, that at least in primary races for the Senate, not only is there no momentum, there isn’t even  trajectory. You see a lift and it’s as likely to go the other direction as it is to continue any trajectory of growth. Is he right?

JOHN SIDES:  Yeah, I do think that’s right. The race that I  probably know the best is the 2012 presidential election, and there you saw several candidates who experienced a pattern, what we call discovery, scrutiny and decline, so the initial discovery where they draw attention in the news media, their poll numbers increase, that look like momentum, at least for a period of a few weeks but then as the scrutiny of their candidacy grew, the coverage became less favorable, other candidates were discovered, and then they experienced a decline.

For other candidates – Romney is a good example from 2012 - the pattern was quite different. You don't see a rise and then a decline. Romney was kind of the tortoise to everyone else's hare. You know, he just stayed steady, no real evidence of a  momentum but it really matter in the end because he became the nominee.

BOB GARFIELD:  And yet, the term persists. It's constantly used by reporters. And this gets to the desires and needs of reporters, themselves.

JOHN SIDES:  You need to be able to talk about something that is changing, and sometimes some of the basic fundamentals of the race really don't change. You know, you can't write a story that says, the poll numbers today were the same as the poll numbers were last week and the week before that and the week before that. So I feel like there is a risk at times to latch onto  something and suggest that one candidates actually is gaining ground. You have to be cautious that you're not just rooting for the story a little bit to give it some oomph.

BOB GARFIELD:  You’re most familiar with the 2012 presidential race. You say that the momentum narrative really got under your skin. Tell me how and why.

JOHN SIDES:  There was a story in Politico that said that all of a sudden Obama's campaign was playing defense, Mitt Romney was  surging, suddenly playing offense all over the map - I'm giving you quotes here from the story. And, in reality, the polls were flat. Obama had lost a little bit and Romney had gained a little bit after that first debate at the beginning of October, but then nothing had changed. I actually complained a little bit to a couple of reporters about this, and one of them went all mea culpa and said, and I’ll quote to you, “What we call momentum is more like narrative and we’re buying into that.”

And what was pretty remarkable for me was that three days after Politico ran that story they ran a second story that said that Romney's momentum, what they called “Big Mo,” had suddenly become “Slo-Mo.” Okay, so all of a sudden we went from Romney has momentum and he’s playing offense all over the map, and then three days later, never mind.

BOB GARFIELD:  It’s one thing to look at a blip in an ongoing campaign and look at the data and mistakenly assume that it suggests some sort of trajectory when, in fact, it does not. That's kind of an understandable error. What's not understandable is to invent momentum out of whole cloth, for the sake of keeping a story alive. Is that malpractice?

JOHN SIDES:  “Malpractice” is such a strong term. How do I want to put this? I look at that as an incomplete job of interpretation. There are a variety of signals that a campaign is sending you. There are lots of polls. There is what you might be seeing on the ground as you travel with candidates. My  objection to that particular story was that we have lots of places now that can average polls together for you and show you a trend line, and the trend line was flat.

You know, the Romney campaign was somewhat famous for getting a little sucked in by the crowds and the emotion of its rallies in the closing days of the campaign, but you got to realize that, you know, just because you can go from city to city and, and 5,000 people show up to cheer for you, that doesn't really necessarily mean anything in a country where 100 and some million people are going to cast votes in an election.

BOB GARFIELD:  Can you give me an example in recent history when momentum really was at play and a candidate did surge through all obstacles to victory?

JOHN SIDES:  The archetypal type case is Jimmy Carter, 1976.

  [CLIP]:

FORMER PRESIDENT JIMMY CARTER:  But since we've come in first now in Maine and Vermont and New Hampshire and Wisconsin and Illinois and Florida and North Carolina and Kansas and I think about seven, eight other states, the attention given to me and the growing enthusiasm of voters has been very encouraging.

  [END CLIP]

JOHN SIDES:  What’s interesting about the Carter story is the way in which the Democratic Party actually changed the rules, so after the 1980 election that created these things called superdelegates, which are basically elected party leaders who have votes at the convention and whose votes are not necessarily determined by the outcomes of primaries and caucuses.

BOB GARFIELD:  Yeah, it was the institutionalization of the smoke-filled room.

JOHN SIDES:  Exactly, kind of a – an attempt to put brakes on a candidate’s momentum, if it came down to it. Parties don't have smoke-filled rooms anymore, but they do things to try to make nominations battles, in particular, not simply a question of media attention and momentum. They try to winnow the field, put resources behind certain candidates, as opposed to other candidates. It’s not been particularly successful for the Republican Party in, in 2016.

BOB GARFIELD:  Yeah, and I guess that leads to my final question, and that is as long as we’re going to [LAUGHS] be discussing Newton's law of motion, we should remember that Newton's third law of motion, for every action there is an equal and opposite reaction. This stuff doesn't happen in a vacuum.

JOHN SIDES:  Yeah, I think that's right. What enables a  candidates to do well in the particular day or week or in a particular state oftentimes means that the opponent will see what happened, adjust their strategy accordingly and maybe find ways to kind of blunt whatever momentum a candidate might get from that victory. And, again, at the end of the day the pattern looks a lot more complex than just, you know, one candidate steamrolling to the nomination.

BOB GARFIELD:  John, thank you very, very much.

JOHN SIDES:  Thank you, Bob, my pleasure.

  [MUSIC UP & UNDER]

BOB GARFIELD:  John Sides is a professor of political science at George Washington University, a blogger for the Washington Post and co-author, with Lynn Vavreck, of The Gamble:  Choice and Chance in the 2012 Presidential Election.

BROOKE GLADSTONE:  Coming up, how to get famous when you're dead.

BOB GARFIELD:  This is On the Media.