WASHINGTON — With the first two debates of the general election finished and early voting starting in many places, Hillary Clinton’s lead has strengthened in opinion polls in key states needed to win the White House, especially those with large minority populations.

But Donald Trump’s core support among blue-collar white voters remains largely intact, giving him a base from which to try to stage a comeback in the campaign’s final phase, judging by a raft of new polls released in the past several days.

A key for Trump is tenacious support among men, whose backing for him increased after Clinton’s health became an issue in early September, according to the USC Dornsife/Los Angeles Times “Daybreak” tracking poll of the race.

Nationally, the race has not shifted dramatically since the first debate between Clinton and Trump even though large majorities of voters say Clinton won the encounter. But beneath that relatively stable national picture, the lineup of states has shifted to Clinton’s benefit, the polls indicate.

The Daybreak poll, a national survey which is updated daily, has shown almost no movement. National surveys by YouGov for the Economist and SurveyMonkey for NBC, both of which poll each week, also have shown a very stable race, although they disagree with the Daybreak poll about which candidate leads.

All three of those surveys are conducted online; several national surveys conducted by telephone have found a larger shift toward Clinton. That sort of difference, with telephone-based surveys having bigger ups and downs than online ones, has shown up in previous campaigns.

The Daybreak tracking poll is the only major survey that shows Trump ahead; as of Wednesday, he leads the poll 47 percent to 43 percent. Averages of recent polls, by contrast, show Clinton leading by four or five percentage points.

One important reason for the difference is how surveys account for people who are not certain to vote or are unsure about which candidate they support.

Clinton and Trump are now even in the Daybreak poll among the most committed voters – those who say they are absolutely certain to vote and sure about which candidate they back. Trump previously had an edge among those voters. His overall lead in the poll now depends on support from people who say they are less than 100 percent sure to vote. The Daybreak poll asks people to estimate how likely they are to vote, using a zero-to-100 scale.

The gender gap provides another big reason for the difference. In the Daybreak poll, Trump leads Clinton by 17 points among men, while Clinton has a nine-point edge among women. Surveys that show Clinton ahead have the gender gap cutting in the other direction – with Clinton’s lead among women exceeding Trump’s margin among men.

The NBC/SurveyMonkey poll, for example, which showed Clinton ahead 50 percent to 44 percent, found her leading by 18 points among women, double the size of Trump’s lead among men. The YouGov survey, which showed Clinton leading 43 percent to 40 percent, had her ahead by 11 points among women, while Trump led by five among men.

Support for Trump from men increased sharply in the Daybreak poll just after Sept. 11, the day Clinton was recorded on cellphone video stumbling into a van while leaving a memorial ceremony for the victims of the 2001 terrorist attack. She later disclosed she had been diagnosed with pneumonia.

Trump has used that episode as evidence that Clinton lacks the stamina to be president, a charge he frequently lobs, and the increase in his support among men in the poll suggests that line of attack may have found a receptive audience. Between Sept. 11 and the end of the month, Trump supporters’ likelihood of voting increased by five percentage points in the survey, with men driving much of the increase.

But Trump’s approach may have further alienated women, a huge stumbling block for him in some key states.

A recent poll of Florida voters by Quinnipiac University, for example, showed Trump losing women in the state by 20 percentage points while winning among men by 11 points. In Pennsylvania, a poll this week by Franklin & Marshall College showed him losing women by 12 points.

The gender gap is one of three huge divides in the electorate that have framed the race since Clinton and Trump secured their parties’ nominations. The other two involve race and education.

The divide along racial lines has been critical in the state polls. Overwhelming support from minority voters has been key to Clinton’s leads in both Florida, where she has led in five out of the last six polls, and in Pennsylvania, a state that Clinton’s campaign has seen as a firewall against any resurgence by Trump.

Minority voters have also been crucial for Clinton in North Carolina, a state that Republican nominee Mitt Romney carried four years ago, but in which Clinton has led in six polls released over the past two weeks. A poll of the state by Elon University found Clinton winning 98 percent of black voters while Trump led among whites 61 percent-39 percent.

In each of those states and nationwide, the education gap has also been a major factor. Trump has dominated the race among white voters who did not graduate from college. He has trailed Clinton in many states among white college graduates, a group that Republicans used to dominate.

The divide between white college graduates and those without a degree has been key to Clinton’s lead in Virginia and Colorado, both states with a high percentage of college-educated voters. Both were hotly contested four years ago, but this year, the Democrats have cut back drastically on their campaign advertising because they don’t think they need to spend much more money to win them.

By contrast, Trump appears to be holding his own in states with whiter electorates and fewer college graduates, notably Ohio and Iowa, two battleground states that President Obama carried in 2008 and 2012 but where Trump now appears to hold narrow leads.

The Clinton campaign has recently stepped up its efforts in other heavily white northern states, including Minnesota and Maine, suggesting concern about soft support in places where the minority vote is smaller than average.

The problem for Trump is that even if he carries those states, it’s not enough to win the White House. To win the 270 electoral votes needed to gain the presidency, Trump almost certainly would need to carry North Carolina and Florida as well as Ohio, and even then would need to take some other state currently in Clinton’s column.

In addition to the divisions along lines of gender, race and education, Trump also is contending with defections by a small, but important, slice of Republican voters in several key states.

In the Quinnipiac poll, for example, Trump was winning 84 percent of Republican voters in Florida, compared with Clinton’s backing from 92 percent of Democrats. In Pennsylvania, the Franklin & Marshall poll showed Clinton getting 78 percent support from Democrats while Trump got 71 percent from Republicans. A sizable share of Republicans, 11 percent in that survey, said they did not know how they would vote.

A small decline in support for third-party candidates may be widening that partisan gap to Clinton’s advantage. The SurveyMonkey poll, which has a very large weekly sample that allows analysis of small groups in the electorate, has shown a decline in support for both the Libertarian candidate, Gary Johnson, and the Green Party nominee, Jill Stein. The Daybreak poll also shows a small increase in the share of voters who say they are certain to support either Trump or Clinton.

Earlier this fall, the third party candidates appeared to be siphoning more voters away from Clinton than Trump. As support for them ebbs, however, voters who lean Democratic may be returning to their usual partisan home faster than those who lean Republican.

That trend could continue. The Franklin & Marshall survey found that fewer than half of Johnson’s voters in Pennsylvania said they were certain to vote for him. By comparison, 92 percent of Clinton’s supporters and 88 percent of Trump’s said they were certain about their choice.