(CNN) Former Vice President Joe Biden welcomed endorsements this week from his former 2020 rival Vermont Sen. Bernie Sanders and former President Barack Obama . After all, Biden needs all the help he can get with a group of voters that Sanders and Obama are popular among: young voters.

Biden, at this point, seems to be underperforming 2016 Democratic presidential nominee Hillary Clinton with that bloc of voters.

The final pre-election polls among registered voters had her leading President Donald Trump among 18-34 year-olds by a 22-point margin, according to a compilation by The New York Times' Nate Cohn . The post-election Cooperative Congressional Election Study with verified voters indicates a similar margin. Either way, Clinton was greatly overperforming among younger voters than her final 2-point national margin.

To see where Biden is, I took an average of the last five high-quality national probability polls with an 18-34-year-old voter breakdown ( ABC News/Washington Post Monmouth University and Quinnipiac University ).

Biden leads Trump by 14 points on average among them. He is, in other words, doing about 10 points worse than Clinton did, even as he is leading Trump by a wider margin (6 points overall) than Clinton was in the closing days of the 2016 campaign.

Biden's weakness with younger voters is something we saw in the primary as well. He averaged less than 20% of the vote among those under the age of 30 in states with an entrance or exit poll . Just looking at those voters who chose between Sanders and himself, he did 5 points worse than Clinton did while competing with Sanders with this bloc four years ago.

Sanders, of course, did considerably better than Biden with those voters this cycle. He won those under the age of 30 in nearly every state and usually by 40-point margins or greater.

Perhaps more important, Sanders seemed to be doing better among younger voters than Biden in the general election as well. Sanders led among those under the age of 35 by 24 points in the most recent Selzer poll. Biden was up just 13 points with them in the same poll. A CNN/SSRS poll from March showed Sanders winning with them by 6 points more than Biden was winning them by.

That's amazing because Biden was doing better than Sanders was overall against Trump in both of those polls.

Now, the good news for Biden is the fact that Sanders was doing better suggests that Biden can win over at least some of these younger voters. Indeed, a lot of them say they are undecided. Looking at those five polls we examined before, 13% of those under 35 say they are undecided or choose none of the above. That's 6 points more than among voters overall in the same polls. This suggests there may be some bitterness against Biden from the primary, which is translating to a higher percentage who don't back either Biden or Trump.

If Biden can win over some of these younger voters, it could help him expand his general election lead. He's already leading nationally by more than Clinton won the 2016 popular vote by, even as he is doing worse among younger voters. Biden is doing so because he's actually winning older voters by 9 points, a group Clinton lost.

If Biden continues to outperform among older voters while underperforming among younger voters, it could potentially make an untraditional map in the electoral college. On that score, we'll have to wait and see.