Joe Biden speaks at the Chuck Hagel Forum in Global Leadership on Feb. 28 at the University of Nebraska at Omaha. (Nati Harnik/AP)

Former vice president Joe Biden is well liked.

You don’t have to take my word for it: Gallup released a poll Thursday that showed 56 percent of Americans viewing him favorably, a decent percentage, given how polarized the country is and given that he’s a potential Democratic presidential candidate. Even 30 percent of Republicans view him favorably, a figure that it seems hard to believe any Democrat could obtain once sitting in the Oval Office.

Among Democrats, of course, he’s even more popular. Gallup has him at 82 percent approval among members of his party, a figure that would prompt President Trump to tweet in all caps. Democratic primary polling also shows Biden in strong position coming out of the gates. Or, really, before even entering the gates, since he’s not yet a candidate.

Monmouth University released a poll last month that showed Biden earning nearly 30 percent of the vote in a field that included 18 candidates.

(Philip Bump/The Washington Post)

In PredictIt’s real-time marketplace allowing people to bet on who is most likely to be the Democratic nominee, Biden has consistently been in the top tier over the past month, along with Sens. Bernie Sanders (I-Vt.) and Kamala D. Harris (D-Calif.), as well as another not-yet-a-candidate, former Texas congressman Beto O’Rourke.

(Philip Bump/The Washington Post)

Sure, Sanders is ahead right now, but he surged once he formally announced his candidacy on Feb. 19. No reason to think that Biden wouldn’t similarly leapfrog ahead.

In 2016, Trump similarly faced a field crowded with qualified opponents. His success stemmed directly from that crowd: By building a fiercely loyal base coming out of the gates — a base constructed from his embrace of right-wing talking points and his hard line on immigration — Trump was able to consistently float above the pack until his opponents fell away and the party coalesced around him.

That’s one theory of Biden, too: He’s beloved by Democrats and already locking down a big chunk of the electorate. How do you defeat that?

The answer is probably simple. First, it’s not clear that Biden’s base is all that big. Second, he starts from a much different position than Trump did in 2016, given his history. And, third, there’s no guarantee that a similar strategy would work for a Democratic candidate four years later.

We’ll start with that polling question. Sure, Monmouth shows him pulling in nearly 1 in 3 Democratic votes. But that’s in Monmouth’s poll, where “undecided” wasn’t a listed option. Nine percent of respondents volunteered that answer, but others who probably haven’t made up their minds picked one of the listed candidates.

Compare that with a poll from Bold Blue Campaigns released last month. In this poll, the question included a specific question asking whether respondents were undecided. Nearly half said they were.

Under those circumstances, Biden’s support dropped to 12 percent — still atop the field but only barely.

(Philip Bump/The Washington Post)

The poll was conducted after Harris announced and before Sanders did, which means that the ordering has probably changed since, but the picture here isn’t that Biden is unstoppable. It’s that he’s in the mix.

A December Quinnipiac University poll showed that Biden was the best-liked potential Democratic candidate — but he was also the best known. Adjust other candidates’ favorability ratings among those who have an opinion, and Biden is not much more popular than O’Rourke, Harris or Sen. Cory Booker (D-N.J.).

(Philip Bump/The Washington Post)

His being well known presents a challenge of its own.

Trump’s announcement in June 2015 established who he was as a political figure. He had been tweeting for years and was known for his embrace of birtherism, but his politics were basically retweets of other people. His speech announcing his candidacy planted his views firmly into the ground — and many Republicans embraced him.

People already understand Biden as a political actor. He was the genial vice president to a president popular among Democrats who enacted policies popular with the base, right? Good position to be in.

We’re already seeing, though, how that can erode.

The Washington Post reported on Thursday that Biden had opposed a school desegregation plan in the 1970s — an issue that Biden embraced at the time to appeal to an electorate that was very different from the Democratic Party of today. Like any long-term politician, Biden has taken firm positions on a number of issues that, in the light of a much more liberal Democratic Party in a much different political moment, will probably sound sour notes.

Hillary Clinton spent a decent amount of time in the 2016 election discussing her vote to authorize the war in Iraq, an issue that would also haunt Biden. During the Supreme Court confirmation process for Brett M. Kavanaugh, Americans were reminded that Biden had overseen the process that confirmed Clarence Thomas — and Biden’s treatment of Anita Hill at that time was almost certainly not something he was happy to have revived. Biden, like other Democrats in the high-crime 1990s, embraced harsh treatment of criminals that is now considered to have been a mistake. Biden hasn’t cast a vote in Congress in a decade; how other votes will hold up now remains to be seen.

It’s worth noting that even that association with former president Barack Obama isn’t necessarily an unalloyed good. In an interview with Politico, freshman congresswoman Ilhan Omar (D-Minn.) offered a view of Obama’s legacy that isn’t uncommon among a Democratic base that’s increasingly rejecting the centrist policies that defined the party during the Bill Clinton era.

“We can’t be only upset with Trump,” Omar said. “… His policies are bad, but many of the people who came before him also had really bad policies. They just were more polished than he was.” She even disparaged Obama’s “hope and change” mantra as hollow — a near-unthinkable assertion for a Democrat a decade ago.

In other words, there’s certainly a case to be made that the popular, front-running Biden of the moment could quickly transform into a less-popular, middle-of-the-pack Biden once Democratic voters contrast his track record with candidates who have embraced positions that are more in step with where the party is today.

Biden seems to embrace the idea of running as a moderate, bipartisan candidate, looking toward how he’ll be positioned for the general election. But he has to get there first.

Let’s say, though, that Biden does lock down about 15 percent of the party as he competes against 10 to 15 other Democrats. Is that enough to win?

Maybe not. It could certainly be the case that the 2020 Democratic field quickly separates out into a handful of front-runners, all of whom have a decent chunk of the electorate in their pockets, and a crowd of candidates further back. The Trump path to the nomination worked for Trump, but, as with so many other things, it may have been specific to Trump.

Biden does have that popularity, though, which Trump certainly didn’t. It’s hard to argue with Gallup putting your favorability that high, including among Republicans.

What Gallup also noticed, though, is that Biden’s favorability had slipped a bit. That happens: When Hillary Clinton left her position at the State Department, her approval dropped from a 2012 high of 66 percent. Even 40 percent of Republicans viewed her positively then.

By early 2015 as she prepared to run for the presidency, her favorability among Democrats was still high, at 79 percent. That was almost exactly four years ago. At the time, Clinton was up by 53 points over Sanders.

It didn’t last.