Let’s just say it: Joe Biden should be expected to win this election. He’s an incumbent president running for re-election with a reasonably healthy economy against an unpopular opponent accused of multiple federal crimes.

And yet President Biden is not winning, at least not now. Polls show him trailing in states worth well over 270 electoral votes, and this morning he lags Donald J. Trump in our newest New York Times/Siena College national poll by five percentage points among registered voters, 48 percent to 43 percent.

That’s the largest lead Mr. Trump has ever had in a Times/Siena national poll. In fact, it’s the largest lead Mr. Trump has held in a Times/Siena or Times/CBS poll since first running for president in 2015.

Why is President Biden losing? There are many possible reasons, including his age, the war in Gaza, the border and lingering concerns over inflation. But ultimately, they add up to something very simple: Mr. Biden is very unpopular. He’s so unpopular that he’s now even less popular than Mr. Trump, who remains every bit as unpopular as he was four years ago.

President Biden’s unpopularity has flipped the expected dynamic of this election. It has turned what looked like a seemingly predictable rematch into a race with no resemblance to the 2020 election, when Mr. Biden was a broadly appealing candidate who was acceptable to the ideologically diverse group of voters who disapproved of Mr. Trump.

Instead, many voters will apparently agonize between two candidates they dislike. It’s exactly what Democrats sought to avoid when they nominated Mr. Biden in 2020. It’s what Democrats largely avoided in the 2018 and 2022 midterm elections, when they mostly nominated acceptable candidates or ran incumbents against right-wing opponents. And it’s exactly what led to the election of Mr. Trump in 2016.

Overall, 19 percent of registered voters in the Times/Siena survey have an unfavorable view of both candidates — a group sometimes referred to as “double haters.” These voters say they backed Mr. Biden by a three-to-one margin among those who voted in 2020, but now he holds the support of less than half. Every vote counts, but these voters will undoubtedly be pivotal in deciding the November election.

The double haters might ultimately return to Mr. Biden’s side. There are still eight months left until November, and it’s not as if these voters like Mr. Trump. If they do come back to Mr. Biden, perhaps their return will have seemed inevitable in retrospect.

But from today’s vantage point, we can’t know what will happen. What we know is that the choice for these voters is much more difficult for them than it was four years ago, when they said they liked Mr. Biden. They don’t today. It creates the conditions for a volatile race, and it might just be enough to flip their preference for president as well.

You can read the full article on the poll here.

A few other items of note:

• In our last poll in December, Mr. Biden led by two points among likely voters, even though he trailed by two among the wider set of registered voters. But in this poll, Mr. Trump holds a four-point lead among likely voters. That’s still better for Mr. Biden than his five-point deficit among registered voters, and it continues a pattern of unusual Biden strength among the likeliest voters, but the difference is no longer enough to give Mr. Biden the lead.

  Mr. Biden’s strength remains relatively concentrated among the most regular voters, as he holds a 46-45 lead among those who have voted in a midterm or a primary. He trails by only two points among those “almost certain to vote.” But many other voters will turn out in a general election, and at least in this particular poll they’re enough to give Mr. Trump a modest lead.

• The poll found Mr. Trump leading Nikki Haley in the Republican primary, 77-20. That’s pretty good for Mr. Trump, of course, but it’s actually Ms. Haley’s best result in a month. And according to our poll, there’s a simple reason for her strength: Biden voters, who now make up 15 percent of those who say they will probably vote in the Republican primary. In fact, a near majority of Ms. Haley’s supporters (48-31) say they voted for Mr. Biden in the last election instead of Mr. Trump.

• Mr. Biden’s support among nonwhite voters keeps sinking. He held just a 49-39 lead among the group, even though nonwhite respondents who voted in the 2020 election said they backed Mr. Biden, 69-21.

• Despite the positive economic news over the last few months, 51 percent of voters still said the economy was “poor.” In a strange way, perhaps that’s good news for Mr. Biden: Maybe his standing will improve if or when voters begin to gain confidence that the economy has turned the corner.

• Even at this late stage, Democrats are still divided over whether Mr. Biden should be the nominee, with 46 percent saying he should be and 45 percent saying he shouldn’t. We didn’t ask whether Mr. Biden should drop out of the race. We considered it — in fact, we discussed it for days — but many respondents may not know the complications involved in a contested convention.