JOHNSTOWN, Pa. — The occupant of 1600 Pennsylvania Avenue increasingly looks as if it could be decided by voters in Pennsylvania towns like this one.

With 21 days until Election Day, President Donald Trump and former Vice President Joe Biden — along with the TV advertising and campaign events meant to help them — are zeroing in on the state, where 20 Electoral College votes could push either one to victory.

Trump, returning to the campaign trail after having tested positive for the coronavirus almost two weeks ago, will be here for a rally at the airport Tuesday — his first trip to Pennsylvania and only second public event outside Washington since his diagnosis.

“He’s our best asset and he’s going to be a big shot in the arm for the campaign," campaign manager Bill Stepien, one of several people in Trump’s inner circle to test positive for coronavirus, said Monday.

Trump has dispatched family members and allies around the country since being sidelined by the virus. Former New York City Mayor Rudy Giuliani was in Luzerne County this weekend and held an “Italians for Trump” event in Philadelphia on Monday.

Biden, meanwhile, has seized on Trump’s absence and ramped up his travel, aiming to focus the campaign on calling out Trump’s mishandling of the pandemic and his contracting of COVID-19 as signs of the president’s disregard for its seriousness. Biden visited Gettysburg, Erie, and Johnstown in the last three weeks, and plans to be in Philadelphia on Thursday for a televised town hall.

In recent weeks both candidates have focused on white working-class voters in more rural parts of the state. Trump hopes to grow that base, while Biden, who polls better in those areas than Hillary Clinton did in 2016, is hoping to erode it.

Political operatives on both sides reiterated the importance of the whole state, which Trump won by less than 1 percentage point four years ago.

Priorities USA, one of the main Democratic Super PACs supporting Biden, told reporters last week that its analysis showed Trump’s narrow path to reelection rides on Pennsylvania.

Priorities rates Pennsylvania as the most likely “tipping point,” the state most likely to deliver the decisive Electoral College vote that seals the election. But as polls show Biden rising and Trump struggling amid the coronavirus pandemic and his catching the virus, the Democratic group sees other routes to victory for Biden, with the former vice president viewed as competitive in states such as Florida, Arizona, and Georgia.

“We continue to see a widening of the path for Joe Biden to 270 Electoral College votes,” Guy Cecil, Priorities' chairman, said in a briefing for reporters Friday. “Biden continues to shore up support and Trump is struggling both to reconcile and to bring new voters into the fold.”

“He is simply running out of time to turn things around,” Cecil added.

While Cecil painted an optimistic picture for Democrats, he cautioned that the race is close enough in key states that small movements could still turn it into a nail-biter.

Pennsylvania is the most hotly contested of the trio of “blue wall” states that turned red in 2016, ahead of Michigan and Wisconsin in terms of campaign attention and spending. Florida is also seen as a must-win for Trump, and a state that could all but end the race if Biden wins it.

Trump’s campaign advisers said Monday that they don’t trust national polling and are encouraged by gains they say they’ve made with African American voters and Catholics and evangelicals, mobilized by the U.S. Supreme Court confirmation hearings for Judge Amy Coney Barrett.

They point to polls that had Clinton up about 10 points in Pennsylvania in mid-October of 2016.

“You look at these polls and you have to ask yourself, what was different then from what is different today?" Trump campaign adviser Corey Lewandowski asked reporters Monday. Lewandowski said internal campaign polling shows “it’s becoming mathematically impossible for Joe Biden to win this campaign.”

Lewandowski did not disclose the numbers that brought him to that conclusion. Virtually all independent polling shows Biden with double-digit leads nationally and smaller but substantial advantages in key swing states. Some Republicans are openly worrying about a landslide loss.

And while both sides agree the race is likely closer than polls show, pollsters have made changes to how they survey political opinions since 2016, doing a better job of sampling white working-class voters, who make up a large portion of Trump’s base.

Both parties have continued pouring resources into Pennsylvania.

Overall in September, the Biden campaign and pro-Biden outside groups outspent Trump and his allies by a 2-1 ratio, $31 million to $15 million, on Pennsylvania broadcast television, cable, and radio, according to data from the ad-tracking firm Advertising Analytics.

In recent weeks, the Trump campaign has cut advertising spending in states like Wisconsin, Michigan, and Ohio, and shifted resources to Pennsylvania.

Even so, Biden maintains a spending advantage in the Keystone State. His campaign and allied outside groups have reserved almost $35 million in airtime in Pennsylvania from now through Election Day, compared with about $16 million booked by the Trump campaign and his allies, according to Advertising Analytics.

America First Action, the main Super PAC supporting Trump, recently added $5.5 million in spending to a previously announced $18.4 million closing push. The new money will be spent on television, radio, and digital advertising, as well as mailings.

Part of the blitz is a new ad criticizing Biden over the Iran nuclear deal. The Philadelphia media market was the first to get the ad, which is targeting swing voters who disapprove of that agreement.

Another push aims to reach Catholic voters in the Scranton area by promoting Trump’s nomination of Judge Barrett to the Supreme Court. Both efforts aim to move small segments of specific voters, said Peter Towey, a senior adviser for America First Action in Pennsylvania.

“In close, tight races, to make a difference between a couple points you need to go find groups of people,” Towey said.

As Trump’s personal favorability ratings have fallen, largely due to his handling of the coronavirus, he’s aimed to downplay it as an illness from which he quickly recovered.

In downtown Johnstown on Monday, Matt Stutt, 68, said he wasn’t worried about Trump’s visit putting anyone at risk.

“Most of it is overblown,” Stutt said. “They turned it into a political thing.”

In Johnstown’s Cambria County, more than 700 people have tested positive for the virus, and at least seven have died. Stutt, who used to be a Democrat, left the party during President Barack Obama’s administration. “The whole party went nuts-o,” Stutt said, adding that he believes the Affordable Care Act, or Obamacare, led to his employer-provided health coverage becoming more expensive.

Trump, he said, was appealing because he “doesn’t take the bulls—."

Aubrey Haselrig, 21, was less welcoming of the president’s rally. She didn’t know whether to believe Trump’s claims that he was no longer contagious. His campaign has said he tested negative for the virus.

“If he’s infected and he’s coming here, that really worries me because we don’t really have healthcare for a lot of the people,” said Haselrig, a student at Pennsylvania Highlands Community College. Haselrig is voting for Biden “reluctantly” because of the tone Trump has set for the country. “He’s just really hateful," she said.

Staff writers Juliana Feliciano Reyes and Andrew Seidman contributed to this article.