WASHINGTON — President Obama traveled to Philadelphia Tuesday and tore into Donald Trump as unfit to succeed him in the White House, the president’s first solo campaign event on behalf of Hillary Clinton and part of a surrogate surge in Pennsylvania that is intended to shore up a crucial bulwark state for the Democratic nominee.

Clinton’s strong post-convention lead in national and many battleground state polls has evaporated over the past three weeks, increasing the stakes in a blue-leaning swing state that is a cornerstone of her Electoral College strategy. The former secretary of state, whose campaign said that she will return to the trail Thursday after recuperating from pneumonia, holds a slight edge over Trump in Pennsylvania, polls show.

Vice President Joe Biden, former president Bill Clinton, and Chelsea Clinton have also stumped for Clinton in Pennsylvania in recent days. Pennsylvania’s working-class voters are seen as pivotal to winning there, a point Obama emphasized at his rally Tuesday as he pointedly criticized Trump.

“This guy who spent 70 years on this earth showing no concern for working people. This guy’s suddenly going to be your champion?’’ Obama said, tie loosened, squinting into the sun. “I mean, he’s spent most of his life trying to stay as far away from working people as he could.’’

Obama praised Clinton as “a leader who’s got real plans to break down barriers and blast through glass ceilings and widen opportunity for every single American.’’ Not directly addressing her health, he called her “tough’’ and “strong’’ and described her as a fighter who would continue his legacy on a number of fronts.

The Trump campaign is doing its own stumping in the Keystone State. The former reality TV star held a rally Tuesday night in Aston, located in one of the four key counties surrounding Philadelphia. On Wednesday, Trump’s running mate, Indiana Governor Mike Pence, will campaign near Scranton, a blue-collar area that Trump has targeted.

Tuesday night, Trump unveiled his plan to make child care more affordable, proposing to guarantee new mothers six weeks of paid maternity leave and offer new incentives for employers to provide their workers child care.

‘‘We need working mothers to be fairly compensated for their work, and to have access to affordable, quality child care for their kids,’’ Trump said in Aston. ‘‘These solutions must update laws passed more than half a century ago when most women were still not in the labor force.’’

Clinton has weathered criticism since she nearly collapsed Sunday from dehydration, an episode caught on video that prompted her campaign to disclose that she had been diagnosed with pneumonia two days earlier. Clinton was already under fire for comments she made at a fund-raiser Friday describing half of Trump’s supporters as being “a basket of deplorables.’’

Obama’s Philadelphia campaign stop capped more than a week of Pennsylvania stumping by high-profile allies on Clinton’s behalf.

Biden, a native son with blue-collar appeal, kicked off the push on Labor Day, appearing with Clinton’s running mate, Virginia Senator Tim Kaine, in Pittsburgh. They attended the Labor Day parade there and appealed to the predominately union audience.

Chelsea Clinton visited Carlisle, State College, and Scranton a few days later, in her first campaign trail appearance since giving birth to her second child. Bill Clinton was in the western part of the state, outside Pittsburgh, on Sept. 9, the same day that Massachusetts Senator Elizabeth Warren rallied voters in Philadelphia for Clinton and Democratic Senate hopeful Katie McGinty.

This saturation of support underscores just how crucial Pennsylvania figures in Democrats’ plans for besting Donald Trump.

Given where the polls are now, Trump has just a few paths to the 270 electoral votes he needs to claim the White House, and the most plausible of these include him winning Pennsylvania, analysts say. Yet it’s the one of a handful of states where Clinton still appears to be leading her Republican rival — though the race has tightened there as elsewhere.

“Pennsylvania really is a Democratic firewall,’’ Pennsylvania-based Democratic strategist Blake Rutherford said. His view of the map is that Trump most likely must win Florida, Ohio, North Carolina, and Pennsylvania, and the latter is where the New York businessman is furthest behind.

Obama’s visit and the flurry of other high-profile campaigners “is very much part of a program to ensure that Pennsylvania stays in the Democratic column, because no matter whatever happens in those three other states, without Pennsylvania, Trump just has a very difficult path to 270,’’ he said.

The most recent Quinnipiac swing state polling shows Clinton up 5 points over Trump in Pennsylvania, beating him 48 percent to 43 percent in a head-to-head matchup. The same poll found the pair essentially tied in Ohio and Florida, and Clinton up a bit in North Carolina.

Republican Congressman Lou Barletta, a Trump supporter who represents a district in the east-central part of the state, says the polls don’t capture the enthusiasm for Trump he sees on the ground of his predominately blue-collar district.

“Donald Trump resonates with people here. His message hits the nail on the head when he talks about the effect that illegal immigration has on the American worker,’’ said Barletta. “It drives down wages, taking jobs away from people — the national security issue it presents.’’ Trump’s messages on the importance of taking care of veterans and building up the military, he said, also attract voters in his part of the state.

Obama appeared in Philadelphia Tuesday as part of a bid to duplicate his victories in the state in 2008 and 2012, which were built on a foundation of minority support in the Democratic stronghold and surrounding area. That could go a long way to ensuring that Trump’s gains with white working-class voters elsewhere in the state won’t be enough to win its 20 electoral votes.

The challenge for Democrats is translating that past support for Obama into a big Philadelphia turnout for Clinton, including among African-American voters and younger voters.

“Can I just say, I am really into electing Hillary Clinton. This is not me just going through the motions here. I really, really, really want to elect Hillary Clinton,’’ Obama told the thousands assembled in front of the Philadelphia Museum of Art. “I need you to work as hard for Hillary as you did for me.’’

Meanwhile, visits by Biden and Bill Clinton, among others, to Pittsburgh and surrounding Allegheny College were aimed at shoring up white working-class support.

The Clinton campaign is also focused on keeping an edge with college-educated white voters and women in the Philadelphia suburbs, groups also considered crucial to winning the state.

Trump has been performing worse than former Massachusetts governor Mitt Romney with both groups.

But the New York real estate mogul has made inroads in the counties surrounding Philadelphia since Clinton posted a strong lead there earlier this summer, said G. Terry Madonna, a political scientist at Franklin & Marshall College.

“The reason he’s made it closer is he’s doing better in the ’burbs. That’s why this race has gotten so much tighter,’’ he said.

Victoria McGrane can be reached at victoria.mcgrane@ globe.com. Follow her on Twitter @vgmac.