US President Joe Biden took fierce aim Thursday at Donald Trump and his "extremist" supporters, labelling them enemies of American democracy in a prime-time address that sought to fire up voters ahead of key midterm elections.

Speaking in Philadelphia, the cradle of US democracy, the president launched an extraordinary assault on those Republicans who embrace Trump's "Make America Great Again" ideology -- and urged his own supporters to fight back.

"Donald Trump and the MAGA Republicans represent an extremism that threatens the very foundations of our republic," thundered Biden, speaking near the spot where the Declaration of Independence and the US Constitution were adopted more than two centuries ago.

"They embrace anger. They thrive on chaos. They live not in the light of truth but in the shadow of lies."

"There is no place for political violence in America. Period. None. Ever," warned the 79-year-old Democrat -- in a reference to last year's assault on the US Capitol by hardline Trump supporters refusing to accept his defeat.

Citing the nationwide assault on abortion rights by hardline conservatives -- and fears for other freedoms from contraception to same-sex marriage -- the US leader charged that "MAGA forces" were "determined to take this country backwards."

With control of Congress in the balance come November, Biden appealed directly to mainstream Republicans to join forces with Democrats and repudiate Trump's brand of politics -- which still holds sway over much of his party.

And he made it clearer than ever that Democrats intended to make the midterms a referendum on Trump, saying the Republican Party was wholly "dominated, driven and intimidated" by the former president and his MAGA agenda.

"And that is a threat to this country," he said, insisting American democracy had to be defended.

"Protect it. Stand up for it," Biden urged.

- 'Semi-fascism' -

Biden's speech -- billed as an address on the "battle for the Soul of the Nation" -- harked back to an article he published in The Atlantic magazine in 2017, after a deadly white nationalist rally in Charlottesville, Virginia that he says spurred his presidential run.

"We are living through a battle for the soul of this nation," Biden wrote then.

After his election in 2020, the veteran politician initially planned to wage this battle through dialogue with moderate Republican lawmakers, and through economic and social policies aimed at the middle class.

But the talk of reconciliation has died down, as polls seem to indicate the Democratic leader is better served by being more aggressive.

Last week, Biden accused Trump's supporters of being consumed by "semi-fascism."

The term sparked indignation in conservative ranks -- with Republican Senate Minority Leader Kevin McCarthy charging that it "vilifies" millions of "hardworking, law-abiding citizens.

"With all due respect Mr President, there's nothing wrong with America's soul," retorted Republican senator and longtime Trump loyalist Lindsey Graham after Biden's speech. "The American people are hurting because of your policies."

A new poll published Thursday by The Wall Street Journal shows that if the midterm elections were held today, 47 percent of eligible voters would cast ballots for Democrats, and 44 percent would vote Republican.

In March, the Republicans had a five-point advantage.

The Democrats are hoping for an upset in November's elections, in which all of the seats in the House of Representatives and a third of the Senate seats are on the ballot. Traditionally, the midterms don't favor the ruling party.

Things have been going well for Biden lately, however, with inflation slowing, a series of his landmark reforms finally pushed through Congress and Trump fighting off a series of criminal investigations. Polls show widespread support for abortion rights, which could put many Republicans on the back foot.

This would be enough to give hope to the Democrats, who are battling to keep their hold on the House and preserve their Senate majority -- or even strengthen it.

And Pennsylvania will be crucial for any of that to happen.

Historically a key battleground state in US politics, the Keystone State will likely prove vital to both parties in the midterms -- and Biden will visit three times this week alone.

Trump is also planning an appearance in the state on Saturday to support his candidate in the Senate race, TV physician Mehmet Oz.