President Joe Biden on Thursday accused Donald Trump and his supporters of threatening the very fabric of America, saying they are bent on destroying US democracy.
Mr Biden gave a combative speech aimed at invigorating voters ahead of crucial midterm elections that will decide control of Congress.
He accused "Maga forces", devotees of Mr Trump's "Make America Great Again" agenda, of being willing to overturn democratic elections.
Mr Biden said they were "determined to take this country backwards" to a time without rights to abortion, privacy, contraception or same-sex marriage, which are supported broadly by Americans.
"They do not believe in the rule of law," he said in a 25-minute speech from Philadelphia, the birthplace of American democracy, where the US Constitution was debated and signed.
"They did not recognise the will of the people and refuse to accept the results of a free election.
"And they're working right now, as I speak, in state after state to give power to decide elections in America to partisans and cronies."
After devoting much of his energy in 2022 to high inflation at home and Russia's invasion of Ukraine, and enduring two bouts of Covid-19 over the summer, Mr Biden has begun lashing out at Trump-aligned Republicans.
The Democratic president, who previously referred to his predecessor only as “the former guy”, has sharpened his speech against Mr Trump and his followers in recent months after the Supreme Court's decision to overturn Roe v Wade.
"Equality and democracy are under assault," Mr Biden said.
Thursday's remarks came after speeches in recent days where he condemned Maga philosophy as "semi-fascism" and assailed Republican threats against the FBI after a search of Mr Trump's Florida home as "sickening."
In a pre-emptive attack before the speech, House Republican leader Kevin McCarthy accused Mr Biden of trying to divide Americans, and blasted the Democrats’ record in Washington, pointing to rising inflation, crime and government spending.
“In the past two years, Joe Biden has launched an assault on the soul of America, on its people, on its laws, on its most sacred values,” Mr McCarthy said.
“He has launched an assault on our democracy. His policies have severely wounded America’s soul, diminished America’s spirit and betrayed America’s trust."
With control of Congress in the balance in November, the president also sought to strike a rousing note, speaking of the "power we have in our own hands to meet those threats".
Mr Biden, 79, appealed directly to mainstream Republicans to join forces with Democrats and repudiate Mr Trump's brand of politics, which holds sway over much of his party.
"For a long time, we've reassured ourselves that American democracy is guaranteed. But it is not. We have to defend it. Protect it. Stand up for it," he urged.
"I'm asking our nation to come together, unite behind the single purpose of defending our democracy, regardless of your ideology."
Mr Biden referred to the 2017 “Unite the Right” rally in Charlottesville, Virginia, which he said spurred him to challenge Mr Trump in 2020.
Independence Hall, the venue for Thursday night's address, is steeped in American history as the site where the Declaration of Independence and the US Constitution were adopted.
An invigorated Mr Biden, speaking more forcefully and clearly than he has in recent months, has had his approval ratings rise slightly in recent months.
They were buoyed by the passage of a social spending bill and bipartisan gun control package, and the authorisation of the strike that killed Al Qaeda leader Ayman Al Zawahiri.
Republicans backed and inspired by Mr Trump have repeated his false claims about the 2020 election being "stolen", with some indicating they would be willing to overturn future electoral results.
At the same time, the former president has been embroiled in a series of investigations since he left office, including a congressional probe into his activity with respect to the January 6, 2021, attack on the Capitol.
He is also under investigation for possible breach of federal laws in his handling and misuse of classified records allegedly taken when departing the White House last year.
Mr Trump and his allies have portrayed the Justice Department's investigation as a politically motivated “witch hunt” supported by Mr Biden and the Democratic Party.
He is expected to deliver his own address in Scranton, Pennsylvania, on Saturday.
Agencies contributed to this report.