Vice President Kamala Harris and former President Donald J. Trump are carrying out a virtual house-to-house hunt for the final few voters who are still up for grabs, guided by months of painstaking research about these elusive Americans.

Inside the Delaware headquarters of Ms. Harris’s campaign, analysts have spent 18 months curating a list of which television shows and podcasts voters consume in the battleground states. Her team has assigned every voter in these states a “contactability score” from 0 to 100 to determine just how hard that person will be to reach — and who is best to deliver her closing message.

The results are guiding Ms. Harris’s media and travel schedule, as well as campaign stops by brand-name supporters. For instance, the movie star Julia Roberts and the basketball great Magic Johnson earned high marks among certain voters, so they have been deployed to swing states.

At Mr. Trump’s headquarters, in South Florida, his team recently refreshed its model of the battleground electorate and found that just 5 percent of voters were still undecided, half as many as in August. The Trump team calls them the “target persuadables” — younger, more racially diverse people with lower incomes who tend to use streaming services and social media. Mr. Trump has made appearance after appearance on those platforms, including on podcasts aimed at young men.

This furious search for a fickle sliver of the country has grown more urgent because the presidential contest is as close as any since the advent of modern polling, with the two candidates nearly deadlocked across the battleground states. The election could now ride on undecided Americans who have unplugged almost entirely from political news — making them tricky to find even for billion-dollar campaigns.

“These people are not super political,” said James Blair, the political director of the Trump campaign, “and so we’re doing non-super-political media.”

In interviews, senior Harris and Trump advisers divulged some details of whom, exactly, they still view as up for grabs. Both see a group that is younger, with a disproportionate share of Black and Latino voters. The Harris campaign believes it can still win over some white college-educated voters, particularly women, who have historically voted Republican but are repelled by Mr. Trump.

An analysis of polling in the battleground states from The New York Times and Siena College found that a mere 3.7 percent of their voters, or about 1.2 million people, were still truly undecided.

The Times analysis closely mirrored what the campaigns describe: a group heavy on younger voters, people of color and those without college degrees. Black voters make up about 21 percent of the undecideds, which helps explain Ms. Harris’s explicit push for them.

A surgical strategy for Harris

The Harris campaign considers its audience of winnable swing voters to be up to 10 percent of voters in battleground states, slightly larger than what the Trump operation sees for itself or the Times polling indicates.

That is because the campaign includes a large number of Republican women who it believes dislike Mr. Trump, particularly on abortion policy, but want to hear Ms. Harris’s message on the economy and the border before they are persuaded.

This strategic thinking has informed the vice president’s campaign speeches.

On Thursday in Wisconsin, she made a direct call to Republicans, particularly those put off by Mr. Trump. She reminded the audience that she had the backing of Liz Cheney, the conservative former congresswoman from Wyoming, whom Ms. Harris is campaigning with in three states on Monday. Last week, Ms. Harris appeared in Pennsylvania with dozens of Republicans.

“I’m campaigning for every vote because I want and intend to be a president for all Americans,” she said. “No matter their political party, where they live or where they get their news.”

Ms. Harris’s recent interview on Fox News came during her most aggressive media tour as the Democratic nominee — just as early voting was beginning in many states.

In one week, she appeared for interviews on “60 Minutes,” “The View,” Stephen Colbert’s late-night show, the “Call Her Daddy” podcast and Howard Stern’s radio show. It was a wide-ranging media blitz designed to break through to voters across her 0 to 100 “contactability” scale, spurred in part by Mr. Trump’s refusal to participate in a second debate.

The Harris campaign methodically measured who saw her on each program. Internal surveys showed that two-thirds of undecided voters in the battleground states had consumed at least some of her interviews during the week. But in a sign of the fragmented media environment, and underscoring the challenge, no single program reached more than one in three of those undecided voters.

“They are very hard to reach — they are not watching traditional news platforms or other programming with large, mainstream audiences,” said Meg Schwenzfeier, the chief analytics officer for the Harris campaign. “To talk to them, we have to take a layered approach — we have to be on TV, nontraditional platforms, door-knocking, billboards, digital ads, mail — everything really.”

The campaign’s data drove its decision to invest in advertising time during Fox News’s daytime programming, when more women are watching than in the evening, when opinion hosts draw an audience that skews male and less persuadable. It also factored into Ms. Harris’s interview on Fox News.

The Harris campaign also used its polls to figure out which moments had helped change minds.

One example came during Ms. Harris’s appearances on “The View” and with Mr. Stern, where she announced a plan to extend Medicare coverage for at-home care for seniors.

Of the more than 100 clips during her media blitz that were tested for their effectiveness at increasing her support, that proposal ranked at the top, a campaign official said.

Trump’s targeted voters

The Trump campaign’s research found that up-for-grabs voters were about six times as likely as other battleground-state voters to be motivated by their views of Israel’s war in Gaza. The campaign also found that undecided voters were less likely to be white than those in the battlegrounds overall and more likely to be Black. About 25 percent of undecided voters are Black, according to the Trump team.

“The fact that they don’t have younger Black men locked in with less than three weeks to Election Day is a big problem for them,” Mr. Blair, the Trump campaign’s political director, said of Ms. Harris and Democrats. “Historically, that would be part of their base.”