Hillary Clinton has regained her clear lead over Bernie Sanders in national polls

Hillary Clinton seems to have regained her dominance over Bernie Sanders after a long and contentious primary battle. And she has the polls to show it.

Back in mid-April, something big appeared to be happening in the Democratic race: Even as Clinton was amassing an insurmountable delegate lead in the state contests, Sanders was pulling neck and neck with her in head-to-head polling among Democratic voters.

But Clinton has since trended back upward. Several new national polls have since found her pulling ahead of Sanders, erasing earlier signs that Sanders was becoming the more popular figure among Democrats.

This chart comes from the Huffington Post polling average:

And this one comes from the RealClearPolitics polling average:

The trend line is unambiguous: As it becomes increasingly clear Clinton will be the nominee, she's also improved her standing among Democratic voters.

Bernie Sanders's national polling surge matches his string of state victories

There's at least one possible explanation political scientists have for the roller coaster ups and downs of Sanders's national polling numbers: Voters like winners.

Usually, people think of the candidates' national popularity as driving how well they do in each state. But this data suggests that the effect could work both ways: By winning states already disposed to vote for them, the candidates can also improve their national popularity.

On April 5, Bernie scored a big 13-point victory over Clinton in Wisconsin. It was the exclamation point on a hot streak, begun on March 22, where he won five of six contests. Sanders added another win in Wyoming on April 9 to go six for seven.

Around this time, Sanders's national polling numbers skyrocketed. On April 8, three days after Wisconsin, the Huffington Post put the Democratic candidates just two points apart. By April 11, RealClearPolitics' polling average said Clinton had just a one-point lead on Sanders. Three polls in mid-April put Sanders actually beating Clinton among Democrats.

Then the calendar shifted to terrain favorable to Clinton. Clinton won a landslide in New York on April 19, and then won four of five states on April 26.

Every national poll of Democrats taken since New York has Clinton ahead by at least four points. All four polls taken since Clinton's April 26 sweep has her ahead by at least nine points, according to the Huffington Post's polling average.

Political scientists: momentum is a real thing

On the one hand, this seems like a straightforward story: Candidates do better in national polls when they're winning states.

"We do have evidence that there is such a thing as momentum. Altering perceptions of electability among voters can be really persuasive," said Adam Seth Levine, a political scientist at Cornell University.

That's in part because voters appear to make their decisions based on relatively little information about their candidate, said Jan Leighley, a political scientist at American University, in an interview earlier in the election cycle.

"If you don't have much water in the bucket, another drip matters a lot," she said. "Those who were disinterested, or minimally interested, their heuristic is: Clinton keeps winning."