“Will this be the thing that causes his voters to abandon him? How much will this hurt his numbers?” are questions that I, as a pollster, have been asked a lot over the past eight years. And the answers are almost always the same: You know, as crazy as it may seem, any one event might not move numbers very much.

That question has almost always been asked of me about Donald Trump. But since last week’s debate, the focus has shifted. I’m now being asked whether President Biden’s terrible performance on stage represents a breaking point with his voters.

The severity of the president’s seeming decline as seen in the debate may be a shock to many, but voters have long been sounding the alarm about Mr. Biden’s stamina and mental acuity. Mr. Biden’s polling was grim before he even set foot in Atlanta.

While the debate felt like a political earthquake, the polls so far have registered only a modest tremor. Mr. Biden was behind before, and he’s behind slightly more now.

As the American people think about the choice before them, pollsters have taken on a new, and in my view largely undeserved, level of importance in the monumental decision that faces Mr. Biden and the Democratic Party. The answer to the question of whether Mr. Biden ought to attempt to stay in office another four years seems obvious, irrespective of the polling data. Yet the Democratic Party appears to be playing the wait-and-see game, hoping polls will give it permission to pull the emergency brake.

A flurry of public polls have for the last few days offered a mixed picture of the debate fallout. CNN found no movement from its Trump-up-six point lead pre-debate, while the Times/Siena poll shows Mr. Biden losing by nine points among registered voters — an eye-popping figure until you realize he was already down by six in a Times/Siena poll before the debate. CBS News’s polling showed a similar decline— from a one-point lead to a three-point deficit — for Mr. Biden in the margin among likely voters in battleground states, relatively modest and within the margin of error.

We saw a similar shift after the first Barack Obama-Mitt Romney debate in 2012. On debate day, Mr. Obama held a three-point lead over Mr. Romney, according to the RealClearPolitics’s polling average, while one week later, the race had swung four and a half points in Mr. Romney’s direction. The polls for the current race could very well continue to worsen for Mr. Biden as Democratic leaders grow increasingly agitated about his being at the top of the ticket.

In considering why the polls have dipped but not completely cratered for Mr. Biden in the week since the debate, a simple explanation comes to mind from a focus group of disappointed Biden voters I conducted.

“Show of hands: Do you think Joe Biden’s up to the task of being president through the year 2028?”

The focus group respondents took a second to process the question. Not a single hand went up. This wasn’t a focus group from this week, assessing the performance of Mr. Biden’s candidacy in the wake of a cataclysmic debate performance; it was from the spring of 2023.

The Democratic strategy for weathering the storm of Mr. Biden’s performance has shifted over time. A day or two after the debate, it was simple: party officials would put their heads down and power through. Voters were told by the campaign and its surrogates that Mr. Biden merely had “a bad night.” Nothing to see here, move along.

Voters, for their part, do not seem to have bought it. No talking points could convince Americans that they did not see what they plainly saw on that stage. Some 72 percent of voters said, soon after the debate, that they did not think Mr. Biden had the mental and cognitive health to serve as president.

Mr. Biden’s standing in the polls pre-debate had already incorporated the pervasive view that he had lost a step. Voters have been more concerned about Mr. Biden’s age than they have been about Mr. Trump’s criminal convictions. They knew that he is over 80 years old.

Voters did not think Mr. Biden had stamina, strength or mental sharpness before debate night. A sizable share of voters were deeply concerned about his mental acuity well before “we beat Medicare” entered the lexicon.

This is why turning to polls to force Mr. Biden’s hand is ill advised. It is hard to know where the point of no return might be for the campaign given his steady and incremental loss of support. Mr. Biden may also be inoculated from a sudden polling decline of more than a few points because of the way our current deep polarization has frozen our politics. For better or worse, Mr. Biden’s candidacy has not been about him so much as him not being Donald Trump. In 2020, Mr. Biden won 62 percent of the voters who said that neither Mr. Biden nor Mr. Trump had the physical and mental health to serve as president. While Mr. Biden lost voters who cast a ballot mostly to support a candidate they preferred, he won decisively among voters who said their vote was more “against” the opponent.

What happened a week ago Thursday is now a test of just how much it would take to make Mr. Biden’s voters break with their candidate. The bottom could, of course, still fall out, and Mr. Biden could leave the race.

But I think back to that May 2023 focus group, where not a single participant thought Mr. Biden was capable of serving another four years. At the end of the session, I asked the group if they intended to vote for Mr. Biden again anyway.

Almost all said, in one way or another, that they would.