Hillary Clinton supporters celebrate after she officially secured the Democratic nomination for president  July 26. (Melina Mara/The Washington Post)

When Quinnipiac University asked voters ages 18 to 34 who they prefer in the presidential race, Hillary Clinton was the runaway favorite. Not runaway in the sense of will-Donald-Trump-get-any-votes-from-millennials-at-all, but runaway in the sense that Clinton had a 21-point lead. In 2012, President Obama won 18- to 29-year-olds by 23 points nationally, so this wasn't a hugely surprising result.

The surprise came when Quinnipiac expanded the field to include Libertarian Gary Johnson and the Green Party's Jill Stein. At that point, Clinton's lead over Trump fell to single digits — and her lead over Johnson was only 2 points.

This was a 24-point collapse of support between a 2- and 4-way contest, a bigger drop than any other age group experienced. And it's certainly cause for concern among Clinton supporters. (The candidate, who we suspect has seen a poll or two in her day, spent some time this week doing her best to woo the youngsters.)

It struck me (rather belatedly) that it was worth testing this same transition among other groups. For example: What happens when you test that change in support specifically among supporters of Bernie Sanders (a group that overlaps heavily with younger voters)? What about if you test those who backed a non-Trump Republican in the primary?

The answer? You see big drop-offs.

As we reported in July, nearly 9 in 10 Sanders primary voters back Clinton in a head-to-head matchup against Trump. When you give them the choice to bail for a third-party candidate, though, a fifth do.

That's not specific to Democrats. Fewer non-TrumpGOP primary voters back Trump than Sanders voters back Clinton — only 77 percent do. (Twice as many non-Trump voters back Clinton as Sanders voters back Trump.) Fully a quarter of that group bails for a third party, given the choice.

That hasn't changed much from August, in a poll in which  Clinton was performing much better. Notice that the non-Trump voters were slightly less likely to back him at that point, and slightly more likely to back Clinton.

Broken out by demographic, we see the extent to which that plunge is pretty unique to younger voters. Nonwhite support for Clinton drops 8 points when all four candidates are polled, but the numbers for Clinton fall 13 points among those under the age of 30. Among most other demographic groups, the declines are fairly even — and fairly modest.

The big plunge in support for Clinton when moving from two to four candidates, then, seems fairly centered in that younger Sanders vote. The plunge among non-Trump voters includes some young voters (Trump loses 9 points of support) but is probably otherwise more broadly distributed. Those big drops for the candidates aren't ideal, but there's at least one reason to think that Clinton's in better shape than Trump in that regard. After all, more people voted against Trump than for him, and their support for the party's nominee is weaker than the support Clinton gets from Sanders backers, even after the switch to four candidates.

Earlier on Thursday, we looked at Clinton's get-out-the-vote operation, a system for ensuring that less frequent voters go to the polls. That includes young people, but Clinton can't simply turn them out to the polls in November. The campaign will also want to remind them who to vote for.