Philadelphia (AFP) - US President Joe Biden touted the rebirth of American infrastructure and manufacturing Thursday in a Pennsylvania trip aimed at boosting Senate hopeful John Fetterman, whose closely watched race could be key to avoiding a Democratic wipeout in the midterm elections.

Biden touched down first in the western city of Pittsburgh, where he plugged his signature infrastructure package with a tour of a newly repaired bridge, before attending an evening fundraiser with Fetterman in Philadelphia.

The national spending spree that Biden's Democrats got through the divided Congress is "the most significant investment" in US history, Biden said on a makeshift podium at the edge of Fern Hollow Bridge.

The bridge, which collapsed eight months ago on a day that Biden happened to be visiting, has almost been rebuilt, serving as a poster child for the White House's policies, which include restoring tens of thousands of bridges around the country.

"There's no better place to talk about rebuilding the backbone of America, the middle class," Biden said.

"I want you to feel the way I do -- pride, pride in what we can do when we work together," he said, referring to the pieces of heavy construction equipment lined up around the building site.

The speech aimed to buoy Democrats in the final run-up to the midterms in three weeks, with Fetterman in one of the key races to holding the Senate.

Fetterman, who greeted Biden on the tarmac in Pittsburgh, is known for his multiple tattoos and a love of hoodies and cargo shorts.

He was once a runaway favorite in the battle against Republican candidate Mehmet Oz, a celebrity TV doctor -- but the race has tightened, reflecting sinking Democratic hopes of maintaining the party's already fragile control of Congress.

The Democrat suffered a stroke in May and the Oz campaign has made his health a major campaign issue, arguing he is medically unfit for office.

Fetterman's doctor released a letter this week stating that he could work "full duty" in public office, but his performance will nonetheless be scrutinized for any signs of physical or cognitive weakness when he faces off against Oz in a debate October 25.

The latest average of polls shows Fetterman's nearly 11-point lead in mid-September whittled down to about five points.

With Biden hampered by approval ratings in the low 40 percent range, some campaigning Democratic candidates have even asked him to keep away.

He has avoided large-scale rallies in favor of smaller policy announcements that he hopes can shift the momentum. Just this week, Biden gave speeches vowing to protect abortion access and explaining his attempts to tamp down high energy costs.

But less than three weeks from voting day, Americans appear to be veering toward the Republican message that Democrats are failing on the economy.

That raises the likelihood of Republicans taking control of at least the House of Representatives and quite possibly the Senate -- ushering in two years of political trench warfare for the White House.

Even just the House would give the increasingly far-right Republican Party the ability to shut down Biden's agenda and -- as prominent figures are already threatening -- attempt impeachment.

Numbers don't add up

A New York Times/Siena poll this week showed that, of likely voters, 26 percent named worries over the economy as their top issue, while 18 percent listed inflation, at its highest rate in four decades.

Even on issues where Biden feels he has a winning hand, there are limits.

During his impassioned speech on abortion, the president tapped into widespread anger over the Supreme Court's decision to overturn the half-century-old Roe v. Wade ruling that enshrined national abortion rights.

Predicting a revolt by women voters at the ballot box, Biden said Republicans "ain't seen nothing yet." 

But the Siena poll showed just five percent of likely voters named abortion as their top issue.

At the Philadelphia fundraiser Thursday, Biden criticized Republicans for having suggested that US funding for Ukraine could be cut if the party makes widely expected gains in the midterm elections.

"These guys don't get it," Biden said. "It's a lot bigger than Ukraine. It's Eastern Europe. It's NATO. It's really serious, serious consequential outcomes."

"They have no sense of American foreign policy," Biden said.

Analysts with Larry Sabato's Crystal Ball election newsletter at the University of Virginia said that after giddy hopes of defying expectations to win this fall, the Democrats seem to be coming back to earth.

"It's just tough for a party to thrive with an unpopular president and with the public having significant concerns about issues, like the economy and inflation," they said Wednesday.