If there were any doubt whether Kamala Harris has transformed this year’s presidential election, this morning’s latest New York Times/Siena College polls put it to rest.

In our first swing state polls since her entry into the race, Vice President Harris leads Donald J. Trump by four points each in Pennsylvania, Michigan and Wisconsin among likely voters. It’s a major shift from previous Times/Siena polls, which found Mr. Trump leading Ms. Harris and President Biden by an average of one or two points each across the same three states.

Sometimes, it can be hard to explain why polls shift from week to week or month to month. In today’s polarized politics, it can even be hard to explain why voters ever shift at all. In this case, it’s easy: Ms. Harris’s entry into the race has upended the fundamentals of this election.

Until now, the basic dynamic of the race was driven by Mr. Biden’s unpopularity. It prevented Democrats from running their usual strategy against Mr. Trump and his MAGA allies: Make an election a referendum on Mr. Trump by running a broadly acceptable candidate. Millions of voters were left with an agonizing choice between two candidates they disliked.

With Ms. Harris riding an extraordinary wave of momentum at the top of the ticket, at least for now, the usual political dynamic of the Trump era has been restored. In the poll, at least 49 percent of likely voters in each of Pennsylvania, Michigan and Wisconsin say they have a favorable view of her, a level neither Ms. Harris nor Mr. Biden obtained in any previous Times/Siena poll this cycle.

Views of Mr. Trump haven’t diminished. In fact, his favorability rating ticked up slightly, to 46 percent across the three states — just enough to represent his highest rating in the history of Times/Siena polling. It’s a tally that might have been enough for a clear lead against Mr. Biden, whose ratings had fallen into the 30s in early July. But for now it’s not enough against the surging Ms. Harris.

One way to think about her position is that she has become something like a “generic” Democrat. This might sound like an insult, but it’s really not. In fact, nothing is more coveted. An unnamed generic candidate — whether it’s a Democrat or a Republican — almost always fares better in the polls than named candidates, who are inevitably burdened by all the imperfections voters learn about in the process of a campaign.

When we polled these three states last October, an unnamed Democrat led Mr. Trump by around 10 points, even as Mr. Trump led Mr. Biden and Ms. Harris by about one point each. The upside of a different, broadly acceptable Democrat was purely hypothetical, of course. There was no guarantee that any real-world Democrat could avoid alienating many of the voters who would prefer to vote for someone other than Mr. Trump. And there was certainly no reason to think Ms. Harris would be such a Democrat, as she was viewed unfavorably by a majority of voters and brought plenty of political baggage from her tenure as vice president and her failed 2020 presidential campaign.

But today, Ms. Harris polls a lot more like that generic, unnamed Democratic presidential nominee. On question after question, the poll finds that voters don’t seem to have any major reservations about her. A majority say that she’s honest and intelligent; that she brings the right kind of change and has the temperament to be president; and that she has a clear vision for the country. A majority doesn’t think she’s too far to the left, either: Only 44 percent of likely voters say she’s too liberal or progressive, compared with 44 percent who say she’s not too far either way and another 6 percent who say she isn’t progressive enough. We didn’t need to ask whether voters thought she was too old to be an effective president.

Whether this will last is an entirely different question. Ms. Harris may be polling like a generic Democrat, but she’ll now be subject to much more scrutiny and attack. To this point, she’s benefited from a few weeks of very favorable media coverage, major endorsements and an outpouring of good will from voters who had been yearning for an alternative to two disliked older candidates. But this period will not last forever, and the question is whether she will retain this kind of support when the going gets tough.

The poll doesn’t offer an indication either way. But the huge swing in opinion on Ms. Harris in just the last few weeks is a reminder that the public doesn’t necessarily have firmly held views about her. If Mr. Trump’s lead over Ms. Harris in earlier polling wasn’t necessarily based on solid views of the vice president, it can’t be assumed that her lead over Mr. Trump stands on firm ground today.