Could unexpected Democratic gains foil a midterm Republican victory?

Joe Biden travels to the battleground states of Wisconsin and Pennsylvania on Monday, determined to reframe America’s midterm elections as a defining choice between democracy and the extremism of Donald Trump.

Fighting for every vote, the US president will mark Labor Day with growing confidence as opinion polls suggest that, while Republicans still have the lead, Democrats now have the momentum heading into the home stretch.

Related: Biden warns US democracy imperiled by Trump and Maga extremists

A string of unlikely legislative victories, combined with fury at the supreme court’s decision to end the constitutional right to abortion, could galvanize Biden’s supporters to turn out in huge numbers in November.

Republicans, for their part, have suffered self-inflicted wounds. Several of their Senate candidates – such as football star Herschel Walker in Georgia – have celebrity but little political pedigree. The prospect of Trump facing criminal prosecution for keeping top secret government papers at his Mar-a-Lago home in Florida has also cast a long shadow.

The midterms are typically a referendum on the incumbent president and his party usually loses seats. But recent events have left Republicans rattled and Democrats dreaming of not only holding the Senate but – unthinkable just a few months ago – retaining their majority in the House of Representatives.

“The good news for us heading into Labor Day is we’re on offense,” said Tom Perez, who was chair of the Democratic National Committee (DNC) during the 2018 midterm elections. “We’re putting points on the board and we’re building momentum. And politics, especially in midterm politics, is all about momentum.”

Neither Biden nor Trump is on the ballot but both loom large in the midterms, offering a replay of the 2020 election and preview of a possible 2024 rematch.

On Monday the current president will speak at Milwaukee’s Laborfest celebration in Wisconsin, where incumbent Ron Johnson, a Trump ally, is the most vulnerable Republican senator up for re-election this cycle.

Biden will also stop in Pittsburgh, host to one of the oldest and biggest Labor Day parades, marking his third visit to Pennsylvania in the space of a week. Democrats John Fetterman and Josh Shapiro are battling TV doctor Mehmet Oz and election denier Doug Mastriano for the Senate and governor’s mansion respectively. Eighteen House seats are also up for grabs in the state.

In remarks in Philadelphia, Pennsylvania, on Thursday night, Biden described “Maga Republicans” – using the acronym for Trump’s “Make America great again” campaign slogan – as representing an extremism that threatens the foundations of the American republic. This followed previous comments in which he likened the Maga philosophy to “semi-fascism”.

Celinda Lake, a veteran Democratic pollster who worked for Biden’s 2020 presidential campaign, said such hardening rhetoric is helping to recast the election as a high-stakes contest between the president’s agenda and a return to Trumpism.

“It makes the return of Trump and Trump Republicans very frightening to people,” she said. “And the idea that if you want to beat Trump in 2024 you have to beat him in 2022 is very salient to Democrats.”

For months, Democrats have been urging Biden to use the full force of his bully pulpit. In focus groups, Lake said, one of the major critiques she hears of Democrats is not that they are too liberal, but that they are “too weak”. Asked to name an animal that describes each party, participants often choose turtle or sloth for Democrats and wolverine or bull for Republicans, she said.

Lake added: “We need to show strength and I think the president is really doing that.”

Biden also used his prime time address to make something of a campaign speech, touting a record that includes “the largest economic recovery package since Franklin Delano Roosevelt … the biggest infrastructure investment since President Dwight D Eisenhower … the most significant gun safety law since President Clinton … the most significant healthcare reforms since President Obama signed the Affordable Care Act … the most important climate initiative ever”.

These victories, despite wafer-thin majorities in Congress, along with rising job numbers and falling gas prices, have given Democrats renewed hope on the campaign trail. But perhaps the biggest gamechanger was the supreme court’s decision in June overturning Roe v Wade, the 1973 ruling that recognised a federal right to abortion.

The supreme court has a 6-3 conservative majority including three justices appointed by Trump. The political backlash became clear when, in conservative Kansas, voters decisively rejected a constitutional amendment that would have nullified a state supreme court decision guaranteeing the right to an abortion.

Democrats are seizing on the issue, stressing that reproductive rights will be on the ballot in November. Republicans have been forced on the defensive, with several candidates in swing states removing hardline anti-abortion language from their campaign websites.

Lake called the supreme court decision a “class 9 earthquake” that has profoundly reshaped the political landscape in ways Republicans failed to foresee.

Allan Lichtman, a history professor at American University in Washington, agreed that it has “tectonically shifted the plates of American politics”, adding: “If you look at the losses in 2018, 2014, 2010, they were more typical midterm losses for the party holding the White House. We didn’t have this kind of burning issue that affected people’s daily lives the way the abortion issue is.”

Abortion bans with no exception for rape or incest are just one example, in Democrats’ telling, of how extreme “Maga Republicans” have become. Democrats’ effort to make the election about Trump, rather than inflation or crime or border security, has received several assists from the former president, who is embroiled in legal battles, holding rallies and dominating headlines as of old.

As the Axios website put it: “GOP midterm candidates – who want to talk solely about the prices of gas and groceries – now must contend with background music that once again is Trump, Trump, Trump.”

Perez, now co-chair of American Bridge 21st Century, a Democratic super Pac, said:“This isn’t just base Democrats coming together, it’s a lot of independent voters who are being reminded on a daily basis why they didn’t vote for Donald Trump.”

Perez pointed to Oz in Pennsylvania, Blake Masters in Arizona and JD Vance in Ohio as far-right Trump-backed nominees who are motivating Democrats and repelling independents. “A secondary force that is giving us wind at our back is the fact that the other side is not only extreme, but they are fielding extreme candidates.”

Some Republicans, including the Senate minority leader, Mitch McConnell, share the concerns. Ed Rogers, a political consultant who worked in the Ronald Reagan and George H W Bush administrations, said: “At the top of the ticket, the governor and the senator in Pennsylvania worry me.

“In Arizona, I’m worried that [Democratic incumbent] Mark Kelly is not a good villain and we’ve got a kind of wacky guy, I’m afraid – he needs to pull out of some wackiness. But what may save us is the border issue. There is zero evidence of anyone listening to Mark Kelly about any moderation at the border.”

Even so, Rogers remains confident that a continuation of the 50-50 split in the Senate is the “worst-case scenario” for Republicans, and that they will “certainly” win the House. He added: “Everybody’s chasing the abortion rabbit and incrementally, that’ll be better for the Democrats. Everybody’s chasing the search warrant down at Mar-a-Lago and this and that.

“Forget all that. Keep your eyes focused on the big four: inflation and the economy; crime and the breakdown of social order, particularly in our cities; the border and immigration out of control; and the breakdown of public education in a lot of jurisdictions of America.”

The election could prove less of a referendum on Biden than on Trump. Should his favoured candidates lose in Arizona, Pennsylvania and elsewhere, and should moderate Republican nominees succeed in states such as New Mexico and Oregon, there is likely to be renewed debate over his influence in the party.

Democrats received more positive omens this week. The party’s Mary Peltola won in a special election over Trump-backed Republican Sarah Palin for the House seat in Alaska. The widely respected Cook Political Report, a non-partisan newsletter, moved the ratings of five House seats in Democrats’ direction.

But given that Democrats have a narrow 221-214 majority in the House, Republicans only need to pick up four seats and, the Cook Political Report estimates, they are likely to pick up roughly three automatically thanks to the redrawing of congressional district boundaries.

Wendy Schiller, a political science professor at Brown University in Providence, Rhode Island, said: “Republicans still go into the midterms with an advantage in House districts, which are gerrymandered to favor one party or the other, and voters always like the opportunity to register their dissatisfaction with the party in power in a midterm election.”

She added: “In the Senate, the upper midwest states of Wisconsin, Ohio and Pennsylvania are the real indicators of party strength nationally. In each of these states, the Senate GOP candidate has high negatives but the GOP has made strong inroads among voters who are not happy with Biden, and anger is a good motivator to get out the vote.

“It is also unclear what turnout rates will be among African American voters this midterm and, without a strong turnout among that voting base, the Democrats will have a difficult time winning there, and in other states like Georgia and North Carolina.”

Related: ‘The US could lose the right to vote’: top official’s democracy warning

The midterms will determine control of Congress for the second half of Biden’s presidency. Democratic majorities could allow him to press on with an ambitious agenda; Republican majorities could effectively turn him into a lame duck. Either scenario will have an impact on the presidential race in 2024.

Lichtman of American University believes that Biden will emerge strengthened. “No matter what happens, expectations are everything and Republicans are not going to live up to expectations,” he said.

“I don’t think they’ll perform as expected in a midterm election and it’s likely that Democrats hold the Senate, which is vastly more important than the House for the simple reason that they confirm all the judges. Biden has a lot of judicial appointments to make to counter the many that Trump did. So if they lose the Senate, it would be a disaster because you would imagine Mitch McConnell’s going to block everything.”