Midterm elections in America are traditionally all about the current president -- whose party rarely comes out well. Joe Biden has another plan for this November: to turn the vote into a referendum on Donald Trump and his "extremist" politics.

The 79-year-old Democrat -- still unpopular though his poll numbers have been creeping back up -- has a catchphrase that says it all: "Don't compare me to the Almighty. Compare me to the alternative."

In other words -- whatever his shortcomings may be, they pale in comparison to the dangers of electing Trump's brand of Republican.

In a prime-time speech Thursday night in Philadelphia, Biden lashed out with rare virulence against Trump and those who embrace his "Make America Great Again" ideology, labeling them a threat to democracy.

"Donald Trump and the MAGA Republicans represent an extremism that threatens the very foundations of our republic," he thundered, warning that the former president's most ardent backers -- those behind last year's assault on the US Capitol -- "thrive on chaos."

"They live not in the light of truth but in the shadow of lies," he said.

74 million people

Following up Friday on his speech -- intended to energize voters with two months to go until the midterms -- Biden insisted he was not tarring all Trump voters, who numbered more than 74 million in 2020, with the same brush.

"I don't consider any Trump supporter to be a threat to the country," he told reporters at the White House. "When people voted for Donald Trump, and support him now, they weren't voting for attacking the Capitol. They weren't voting for overruling an election."

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

The New York Times summed up the strategy like this: "If Americans are asked whether they support Mr. Biden, they may say no.

"If they are asked whether they support him over Mr. Trump, they may say yes. At least, that is the theory in the White House."

In that spirit, the White House laid the symbolism on thick in staging Biden's speech -- delivered near the spot where the US Constitution was adopted more than two centuries ago, with dramatic deep red lighting, and two Marines posted behind the commander-in-chief.

The military overtones for what was ostensibly a political speech ahead of the midterms raised eyebrows in conservative circles and beyond.

A White House spokesman, Andrew Bates, pushed back on Twitter, saying Biden's "accurate warnings are the opposite of 'political.'"

But the fact remains that by steering the conversation heavily towards themes such as the defense of democracy or abortion rights, Biden is undercutting the Republicans' preferred talking points on crime and the economy.

Polarization

The Republican leadership has accused Biden of sharpening national divisions with his rhetoric -- and is trying to make that an angle of attack.

But some of the party's fierier members are upping the ante -- like the pro-Trump lawmaker Marjorie Taylor Greene who shared a video doctored to make Biden look like Adolf Hitler.

For Wendy Schiller-Kalunian, a political scientist at Brown University, the Democratic strategy comes fraught with risks.

The November midterms will determine whether the president's party loses control of both the Senate and House of Representatives, whether it manages to hold onto the upper house, or defies the odds by clinging onto both chambers of Congress.

"One of the key groups of voters in this year's midterm election are suburban Republican women, and independent men," who tend to lean right, Schiller-Kalunian explained.

"So if Biden makes this all about Trump and the danger of Republican control of Congress, that may backfire and inspire these suburban voters to support their own 'team,'" she said.

Samuel Goldman, a political science professor at George Washington University, argues meanwhile that "undecided voters make decisions on the basis of concrete issues, particularly the economy."

In that light, Goldman believes Biden's main priority is to "fire up Democratic partisans."

And he says Biden's speech highlighted a dilemma common to all American presidents: to be both the head of state, rising above the fray to speak to all citizens, and the leader of a party seeking victory at the ballot box.

"Due to ideological polarization, media fragmentation, and declining trust in institutions, it's getting harder and harder to play both roles at the same time," Goldman said.

© Agence France-Presse