Sen. Marco Rubio (R-FL) and Donald Trump participate in a debate sponsored by Fox News at the Fox Theatre on March 3, 2016 in Detroit, Michigan. (Chip Somodevilla/Getty Images)

Hillary Clinton came into the 2016 race as the prohibitive favorite to win the Democratic nomination, if not the presidency. In polling taken when she entered the race in the spring of 2015, she led Bernie Sanders by more than 50 points. Over the next twelve months, her lead collapsed; by April of this year, she and Sanders were within a percentage point.

The reason for that was two-fold. A quarter of younger voters preferred Sanders in July 2015 polling from The Post and ABC News. By September, it was a third; by October, 43 percent. In January, it was two-thirds and in May, three-quarters. By the time voting started, Sanders also regularly earned a majority of the support of independents voting in the Democratic primary.

Donald Trump's path to the nomination was different. He quickly built a fervent base of support among working-class white voters after high-profile fights on his immigration position. That core of support stuck with him, and in a field of more than a dozen candidates, it kept him at the top. As voting began, he grabbed enough support from voters who'd been planning to vote for someone else to stay at the front of the pack. He won the nomination with a record number of votes -- but more Republicans voted for someone else than voted for him.

Those same dynamics appear to be making the general election race just as close at the moment as the primaries were at their peak.

A New York Times-CBS News poll released on Thursday showed Clinton leading Trump by 2 points in a head-to-head contest, but the two even in a contest when Libertarian candidate Gary Johnson and the Green Party's Jill Stein are added to the mix. It's not uncommon that the race is closer when the third-party candidates are added. The Post's 50-state survey released earlier this month suggested that Trump was the main beneficiary of having more candidates on the ballot. In the RealClearPolitics polling average since the conventions, Clinton's lead has been an average of 0.7 points narrower in four-way polling.

At this moment, about 10 percent of the electorate isn't picking Trump or Clinton in a head-to-head match-up, versus about 7 percent in the past three elections.

Part of that is due to the unpopularity of both candidates, which suggests that the role of third-party candidates this year may be larger than in years past. We've noted that third-party polling is often higher than the eventual result, but there's enough variability in the small sample of past elections and in this cycle to raise an eyebrow. We also now know that Johnson is expected to be on the ballot in every state, and Stein in most.

Cutting to the chase: Donald Trump is probably benefitting from a four-person presidential field. In four-way polling, Trump's never been above 40.3 percent in the polling average since July 1 -- but Clinton's never been above 44. Trump's ceiling may be high enough if the field is splintered.

Particularly because the field is splintered in a way that hurts Clinton. On Wednesday, Quinnipiac University released a survey showing Clinton with a lead in the national polls of 5 points. But buried in the data was an interesting bit of data: Among young voters, who were key to President Obama's two victories, Clinton is barely outperforming Johnson.

Clinton got more than half of the vote from younger voters in a two-way contest. Add Stein and Johnson to the mix and she gets less than a third. Her support falls by over 20 points, far more than any other age group shifts once the third-party candidates are added to the mix.

The Times-CBS poll found the same thing:

The third-party candidates draw their strongest support from younger voters. Twenty-six percent of voters ages 18 to 29 say they plan to vote for Mr. Johnson, and another 10 percent back Ms. Stein. A little more than one in five political independents say they will vote for one of the third-party candidates.

This was the case that Sanders tried to make to superdelegates after it was already clear that he would lose the primary contest: Hand me the nomination because only I can win younger voters and independents in the fall. At the time it seemed unlikely that Trump would pick up the support of those voters, and, in fact, he didn't. The problem, as in the primary, is that younger, independent-minded voters are looking somewhere besides Clinton when given the choice.

Trump's core base of support, those working-class white voters, haven't budged an inch. He's not doing as well with whites with college degrees, and soft support from that group a month ago was a large part of why he was doing worse against Clinton. He's regained a lot of that ground.

It's critical to note, though, that other factors from the primaries continue, too. Nonwhite voters, for example, still broadly favor Clinton's candidacy, providing a critical buffer. A faction of Republicans finds Trump unacceptable as the nominee, which may be one of the reasons that he's likely to continue to see a soft ceiling on his support. On top of that is the inherent advantage the Democrat has in an electoral college match-up, which we covered yesterday.

If you're looking for an explanation of why Trump and Clinton are running even in a four-way contest, though, it appears to be pretty simple.

Largely for the same reasons that the primaries unfolded the way they did.