Bernie Sanders says he's got it. So does Ted Cruz. But will their victories in recent primaries translate into a big win in next week's New York contest? Will undecided voters jump on the bandwagon, so to speak, and is the psychological effect some call political momentum real or just imagined?
Social scientists say the idea of "social herding" is a long-standing one and does have some observational and experimental basis. People often gravitate to things that are popular precisely because they are popular. That includes consumer products as well as political candidates.
Neil Malhotra is a professor of political economy at Stanford University. He's written about political momentum and the bandwagon effect. He says it often occurs during primary elections when voters' political identities haven't hardened into either Democratic or Republican party affiliations.
"Voters don't have as much to go on in the primaries," Malhotra said. "There's no incumbent and party IDs are the same."
In general elections, the number of undecided voters has shrunk considerably, making it more difficult to take advantage of momentum.
"There's not many people on the margins that could go either way," he said.
Malhotra believes there are several reasons why primary voters may pick the front-runner.
"The first is the idea of normative social influence, you don't want to be excluded so you conform to not be isolated," he said. Call it peer pressure.
"The second mechanism is informational social influence; if a lot of people are doing something, you make the inference that its probably a good thing to do," he added.
The third reason is something called cognitive dissonance reduction.
"It's the story of the sour grapes," Malhotra explained. "I can't get them, so I say I don't want them. Instead, I turned the sour grapes into a sweet lemon. You change your mind and convince yourself to support a candidate because they are ahead. You can convince yourself that you really like them."
Sanders, who was far behind Democratic front-runner Hillary Clinton in public opinion polls last fall and even after the initial primaries in January, has narrowed the gap with victories in seven of the last eight primaries. However, those wins were in smaller states with fewer minority voters, who are traditional supporters of Clinton. Clinton still holds a commanding lead in pledged delegates for the Democratic nomination.
"He's the guy that we would say there has been a momentum swing for them," said Patrick Kenney, dean of the college of liberal arts at Arizona State University and a political scientist who has also written on the bandwagon effect in elections.
On the Republican side, however, where Donald Trump continues to lead despite recent gains by Cruz, Kenney said social scientists are a bit stumped.
"We have no good explanation for Trump," Kenney said. "We will have to wait and see the data on it."
Kenney said that Cruz's momentum in a few recent states may be coming from "strategic voters" who don't necessarily support Cruz's policy ideas, but are worried that Trump would be a worse choice.
Kenney said that American voters are often swayed during primaries because most do not follow politics as closely as voters in other countries. Political momentum gets a boost by the emotional attachments formed by some voters with a candidate, something that may be bringing in younger, first-time voters to Sanders' campaign.
"People who love democracy would like to see more strategic or policy voting, but most data shows that isn't the case," Kenney said. "Americans on average are not big daily consumers of politics. Things pop up for them and they have to make a decision. They're not interested in it. The party is such an easy cue, but in primary, there's no party there."