The Democratic anxiety that has swirled around President Biden for over a year has kicked into overdrive in recent weeks, as his approval ratings have stayed stubbornly low and polls have shown the possibility of his losing to former President Donald J. Trump.

That anxiety has crystallized into one question, repeated like a drumbeat: Can’t some big-name Democrat challenge him? Someone more prominent than Marianne Williamson or Dean Phillips?

The answer: In theory, sure. In practice, the prospects are remote.

There are several reasons for that, most of which boil down to it being really hard to run a successful primary campaign against a sitting president. And doing so at this point, just two months before voting starts, wouldn’t be feasible anyhow.

Making things still more difficult for a would-be challenger is that Mr. Biden remains relatively popular among Democratic voters. According to a recent New York Times/Siena College poll, 79 percent of party voters in six battleground states somewhat or strongly approve of his performance, which doesn’t leave a lot of room for another Democrat.