Some of these voters may support impeachment on the merits but have questioned whether Congress should pursue it if it is unpopular or unlikely to result in Mr. Trump’s removal from office. Others might have assumed that the case for impeachment was not strong, given that the party’s elected officials have generally not supported it. They might see it differently now that support from congressional Democrats is all but unanimous.

Alone, these Democratic-leaning voters could move the polls to something close to majority support for impeachment. At best for Democrats, perhaps these voters by themselves could bring the polls to something near where they were in the Supreme Court nomination of Brett Kavanaugh, when a slight majority of voters opposed his confirmation.

It is also possible that these easy Democratic targets would leave impeachment well short of majority support. So long as that’s true, there may be some political upside for Republicans.

Whether Democrats can move support higher will probably depend on the allegations themselves. Most voters probably have a very high threshold for supporting impeachment and removal from office, as was the case for President Bill Clinton in 1998. They might think it requires conclusive evidence of criminal activity. Even that may be insufficient.

It is hard to know whether the Ukraine allegations will cross the threshold for a majority of voters. One piece of evidence suggests that it might: A YouGov poll, conducted on Monday, found that 55 percent would support impeachment if “President Donald Trump suspended military aid to Ukraine in order to incentivize the country’s officials to investigate” Joe Biden and his son.

Of course, it remains to be seen whether the hypothetical situation laid out in the poll will prove to be true, and persuasive to most voters. The president, along with many Republican allies, will argue it is an insufficient basis for impeachment. They will probably convince some number of Mr. Trump’s supporters of that.

But the poll at least suggests that the allegations have the potential to move public support for impeachment.