Former vice president Joe Biden launched his bid for the 2020 Democratic presidential nomination by squarely focusing on the opponent he was most worried about: President Trump.

It’s a familiar tactic in politics: Make a statement about your confidence of winning the primary by aiming at the next opponent down the line. This was Hillary Clinton’s strategy for much of the 2016 primaries. She trained very little fire at Sen. Bernie Sanders (I-Vt.) and instead focused on Trump and the Republicans. It worked, although by less of a margin than one might have expected.

In the 2020 campaign, Biden’s tactic was probably more justifiable than is usually the case. After all, polls have shown that Democrats are awfully worried about beating Trump, more so, in some polls, than they are concerned about the specific policy proposals offered by possible Democratic nominees.

Biden’s current lead in the Democratic field is a function of a few overlapping factors. He consistently polls better among moderates than his opponents do, scooping up a lot of support from a still-sizable group in the Democratic electorate. In most polls, he gets more support from black Democrats than white ones. That he’s consistently been seen as the candidate with the best shot at beating Trump also plays some role. Sure, voters’ assessments of electability are suspect — before his “oops” moment in a 2011 debate, then-Texas Gov. Rick Perry was viewed as the most electable Republican — but they clearly drive some support, at least in this election.

That last point is why last week’s Democratic debate was potentially so damaging to Biden. On Monday, HuffPost and polling partner YouGov released analysis of how leading Democrats were viewed after the contest. Biden has gotten generally low marks for his performance, in part because Sen. Kamala D. Harris (D-Calif.) effectively raised questions about the former vice president’s views on race.

The result? While Biden enjoyed a 25-point lead on the question of electability in May, he now leads Sen. Elizabeth Warren (D-Mass.), who also had a good debate, by only 6. (The HuffPost question asked respondents whether they thought individual candidates were capable of beating Trump.) Biden is now in within-the-margin-of-error territory against several other leading Democrats. Each, in other words, is seen as about as capable of beating Trump.

This is not great news for Biden — but at least he’s not former congressman Beto O’Rourke (D-Tex.).

The thing that will most shape perceptions of electability, of course, is actual elections. And here we have another warning for Biden.

There’s been some question about the extent to which black support for Biden is driven by perceptions of electability. (Black Democrats are also more politically moderate than white Democrats.) If his support from black Democrats is linked to the question of electability, he might want to be more worried about what happened to Clinton in 2008.

That year, Clinton was consistently seen as the likely Democratic nominee, though to a lesser extent than three years ago. She was also seen as the more electable candidate among black Democratic primary voters — at least until Barack Obama won the Iowa caucuses. (In Post-ABC News polling at that point, the question of electability pitted the two candidates against each other, asking who was more electable.) After Iowa, black respondents were split between the two on the question of electability.

But the effect on Obama’s support was much more dramatic. Before Iowa, Clinton led Obama among black primary voters, albeit narrowly. Afterward, Obama was the overwhelming favorite of black primary voters.

It wasn’t just our polling that showed that effect. CNN’s polls saw a 52-point swing in the relative support for Obama and Clinton among black poll respondents from October 2007 to January 2008.

At this early point, it would be foolish to make too many predictions about how the 2020 race will turn out. (As 2016 taught us, making any predictions before polls close in California is probably risky.) But there are certainly existing signs and historical data showing that Biden’s path to the nomination might be less inevitable than his campaign launch video suggested.