US President Joe Biden heads Thursday to Philadelphia, the cradle of American democracy, to deliver a rare primetime speech slamming Donald Trump's hardcore supporters and the "extremist threat" they pose to the nation ahead of key midterm elections.

Biden is looking to give his Democratic Party a boost before voters go to the polls in November, with control of Congress for the remainder of his first term in the balance.

Pennsylvania, historically a key battleground state in US politics, will likely prove crucial to both parties in the midterms.

Biden will visit the Keystone State three times this week alone, and his speech -- set for 8:00 pm (0000 GMT), will take place near the building where the Declaration of Independence and the US Constitution were adopted more than two centuries ago.

White House spokeswoman Karine Jean-Pierre has already signaled that the 79-year-old leader will not mince words when it comes to his opponents.

"The president thinks that there is an extremist threat to our democracy," she told reporters Wednesday.

And the threat has a name: "MAGA" or "ultra-MAGA" Republicans who embrace former president Donald Trump's "Make America Great Again" ideology.

"They just don't respect the rule of law," Jean-Pierre said, calling out several Republican officials and lawmakers by name for urging violence against public figures, a highly unusual move from the podium in the White House briefing room.

"The president believes, which is a reason to have this in primetime, that there are... a majority of Americans who believe that we need to... save the core values of our country," said Jean-Pierre.

The theme of Biden's speech Thursday -- the "Soul of the Nation" -- harks 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.

- 'Semi-fascism' -

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 rhetoric of reconciliation has died down, as Biden faces a Republican Party still heavily influenced by Trump and polls seem to indicate that the Democratic leader is better served by being more aggressive.

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

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, 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 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.

Biden already traveled to Pennsylvania on Tuesday, and he will return there on Monday to celebrate Labor Day with Democratic Senate hopeful John Fetterman.

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