The lights dimmed, the music throbbed and cellphone lights danced across the arena. Then a DJ welcomed to the stage the president of the United States, Joe Biden, flanked by the former president Barack Obama and Pennsylvania’s nominees for Senate and governor. An ecstatic crowd of thousands roared to their feet.

With days left until the midterm elections, the presidents were in Philadelphia to mobilize Democrats in a pivotal swing state that could determine Congress’ balance of power. But the event also had the feel of a political homecoming for Biden, joined by his former running mate in the state where he was born at the end of a volatile campaign season.

“It’s good to be home,” Biden thundered above the cheering. “It’s good to be with family.”

The president has remained outwardly optimistic about his party’s prospects in Tuesday’s elections, and the Democrats’ electric reception at Temple University’s Liacouras Center on Saturday no doubt gave him even more reason for hope. But nationally, Democrats’ mood had darkened.

After a summertime peak, the party in power is now struggling to overcome historical headwinds and widespread economic discontent. Public polls have tightened in recent weeks. Democrats are now on the defensive in places they thought were safe, like New York and Washington. And Biden’s low approval ratings continue to burden his party’s most vulnerable candidates, many of whom have sought to avoid the president.

Not in Pennsylvania.

On Saturday, Biden clasped hands with John Fetterman – the Democratic nominee for Senate locked in a narrow race that could decide control of the chamber – and Josh Shapiro, the party’s nominee for governor.

Pennsylvania lies at the heart of Democrats’ efforts of staving off major losses in the House, as the president’s party traditionally does in midterm elections, and keeping their narrowest of majorities in the Senate.

Biden declared the midterms “one of the most important elections in our lifetime”.

Hanging in the balance, Biden charged, was the very American experiment that began in Philadelphia nearly two and a half centuries ago, now at risk of falling victim to the cynical forces seeking to undermine the nation’s system of government with lies and conspiracies. In impassioned bursts, he warned of the dangers of electing candidates who have denied the results of the 2020 election and who he says threaten the security of future ones.

“This isn’t a referendum this year,” he said. “It’s a choice – a choice between two vastly different visions of America.”

Making an equally dire case for the Republican party was Biden’s predecessor and political rival, Donald Trump, who addressed a crowd of thousands at an event in the Pittsburgh exurb of Latrobe.

There he reprised familiar warnings of worsening crime, open borders and war on “your coal” – a jab at Biden’s comments from a day earlier pledging to shut down coal plants “all across America” that set off an unwelcome political firestorm within his own party. He also teased a long-anticipated third presidential run: “I promise you, in the very next – very, very, very short period of time, you’re going to be so happy.”

Biden has said publicly he intends to run again in 2024 but has not made a formal announcement. His team have begun preparations for a possible re-election bid though his age and low approval ratings remain a concern for many Democrats.

The convergence of three presidents in Pennsylvania on Saturday underscored the state’s importance as a battleground. In a potential 2024 rematch between Trump and Biden, Pennsylvania’s 20 electoral votes are once again likely to play a decisive role in determining the victor.

Biden on Saturday reminded Pennsylvanians of that power. In 2008, the state helped elect the nation’s first Black president in 2008. In 2020, he said, Pennsylvania elected “a son from Scranton president” and helped make Trump not only a former president but a “defeated president”.

Despite some fretting that Biden’s appearance in Philadelphia might do more harm than good for Democrats in tight races, Biden arrived as the native son.

Though he built his political career in Delaware, Biden’s political identity is rooted in Pennsylvania. And on Saturday he proudly recalled that as a senator from Delaware he was often referred to as “Pennsylvania’s third senator”.

He anchored his 2020 campaign in Philadelphia. As president, he has returned to Pennsylvania on as many as 20 occasions, including a trip to Scranton to tout his infrastructure plan at an electric trolly museum and, more recently, to deliver a primetime address in Philadelphia warning that Trump and his Republican followers “represent an extremism that threatens the very foundations of our republic”.

Biden touted his home state ties to make the case for electing Fetterman to the Senate, saying: “I know Pennsylvania well and John Fetterman is Pennsylvania.”

Then he turned on Fetterman’s Republican opponent, the Trump-backed celebrity doctor, Mehmet Oz, casting him as a carpetbagger from neighboring New Jersey. “Look,” he said, “I lived in Pennsylvania longer than Oz has lived in Pennsylvania – and I moved away when I was 10 years old.”

Tens of millions of Americans have already cast their ballots, though polls officially close on Tuesday and it could take days – or weeks in some cases – to know the final result of an election Biden said will “shape our country for decades to come”.

In the final months of the midterm cycle, Biden has largely avoided states with the most competitive contests, like Georgia, Michigan, Wisconsin, Nevada and Arizona, even though all of them helped elevate him to the White House. Instead, it has been Obama rallying Democrats in those battlegrounds – a role reversal from 2010 when Obama was the unpopular president and Biden, then his vice-president, was the party’s in-demand surrogate.

Yet Biden has kept a frenetic pace on the campaign trail in the final days. On Tuesday, he traveled to Florida, a battleground where Democrats have seen their hopes fade in recent elections cycle, before heading to New Mexico, California and Illinois, Democratic strongholds with competitive midterm contests.

On Sunday, Biden returned to New York, where the race for governor has narrowed in a worrying sign for Democrats’ fortunes elsewhere, and he will headline a rally the night before the election in Maryland.

In his appearances, Biden has tried to rally supporters around his administration’s policy achievements, highlighting initiatives to lower the cost of prescription drugs, boost domestic manufacturing, combat climate change and forgive student loan debt while warning that Republican control of Congress would threaten social security and Medicare.

The economy and inflation consistently rank as voters’ top concern this election, along with crime, abortion and threats to democracy. Democrats have sought to blunt Republicans’ advantage on the economy and crime by arguing that their opponents would pursue an extreme agenda on issues like abortion, guns and voting rights. They have pointed to the threats posed by election deniers loyal to Trump.

“You see these guys standing there with rifles, outside polling places?” Biden said on Saturday. “Come on. Where the hell do you think you are?”

For Democrats to remain competitive Tuesday, their task will be to rebuild the coalition responsible for Democratic victories during the Trump era. They must recapture support from a mix of college-educated suburban voters and Republican-leaning moderates while motivating Black voters and young people to turn out in strong numbers.

Should they fall short, Biden has been blunt about the challenges of governing with Republican majorities. “If we lose the House and Senate,” he said in Chicago, “it’s going to be a horrible two years.”

Taking the stage last on Saturday, in a slot typically reserved for the current president, Obama said he knew all too well what Democrats stood to lose if Biden no longer had majorities in Congress.

“When I was president, I got my butt whooped in midterm elections,” Obama recalled of the 2010 elections. “Midterms are no joke.”

He asked the audience to imagine what it might have been like if Democrats had kept control of Congress. They might have acted on immigration reform, gun safety and the climate crisis. Had they kept the Senate in 2014, he continued, the makeup of the supreme court might look very different. The audience groaned at the thought.

History didn’t have to repeat itself, Obama said. Democrats didn’t have to imagine what Biden could accomplish with another majority in Congress.

“The good news is, you have an outstanding president right now in the White House,” Obama said, ticking through Biden’s legislative accomplishments.

“You’ve seen what he’s accomplished with the barest of margins,” he said. “If you vote, he can do even more. But it depends on you.”