Biden argues Trump is an existential threat to America; Trump tries to point finger back at Biden

President Joe Biden on Tuesday labeled rival Donald Trump an existential threat to the country's system of government as he responded to the former president's recent comments pushing back on such criticism by claiming it is Biden who is really the "destroyer of American democracy."

The back and forth reflects how the issue of protecting democracy and Trump's baseless attacks on the 2020 election could be key in 2024.

"I don't think anyone doubts our democracy is even more at risk [than in] 2020," Biden said at the first of three campaign fundraisers in Boston on Tuesday, adding, "This time, we're running against the election-denier-in-chief."

"Trump's not even hiding the ball anymore -- he's telling us what he's going to do, he's making no bones about it," Biden said.

Later on Tuesday, as he continued his swing of fundraisers, he also said, "If Trump wasn't running, I'm not sure I'd be running" -- though he said the opposite to ABC News Chief White House Correspondent Mary Bruce earlier this year.

"I still think I would be running if he wasn't," Biden said then.

On Tuesday, Biden warned Trump was a major threat, in his words: "Let's be clear about what's at stake in 2020 ... Donald Trump and his MAGA Republican determined to destroy American democracy."

But, he said, "Don't take my word for it: Listen to what he's actually saying these days. He said it out loud. He says 2024 is the 'final battle.'"

Biden was referring to Trump's rhetoric on the campaign trail in which he argues that unless he is elected back to the White House, "I truly believe our country is doomed," as he said in March in Texas.

The president, who looks increasingly likely to be headed for a rematch against Trump next November, has sought to define the 2024 presidential election in terms of protecting not just democracy from Trump -- but personal liberty.

In a video earlier this year announcing his reelection bid, Biden said, "Every generation of Americans has faced a moment when they've had to defend democracy. Stand up for our personal freedoms. Stand up for the right to vote and our civil rights. And this is our moment. Let's finish this job, I know we can."

On Tuesday, Biden attacked Trump for the latter's role in ending Roe v. Wade by naming three Supreme Court justices, all of whom joined an opinion in 2022 that reversed the national guarantee for abortion access.

"He's proud to say he killed Roe v. Wade," Biden said, adding, "He's running again to get rid of the Affordable Care Act."

Of Roe, Trump has said on social media, "After 50 years of failure, with nobody coming even close, I was able to kill Roe v. Wade, much to the 'shock' of everyone. ... Without me the pro Life movement would have just kept losing."

On the Affordable Care Act, also known as Obamacare, Trump recently contended it is a "disaster" and is going to "give much better health care than what you have right now."

As he has in previous fundraisers, Biden on Tuesday raised Trump calling his political enemies "vermin," saying immigrants in the country without documentation were "poisoning the blood of our country" and vowing to represent "retribution."

"You don't expect to hear a president say these things, but he is -- and out loud," said Biden, noting that Trump has previously indicated he would give presidential pardons to many of the people charged or convicted in connection with the Jan. 6 attack on the U.S. Capitol as Congress had gathered inside to certify Biden's election victory.

Biden also referred to Trump last year suggesting that some of the Constitution should be terminated because of his unfounded claims of widespread election fraud. Trump later insisted he was being misunderstood.

"I've never walked away from what Trump wants to do. We're not going to walk away now," Biden said at the fundraiser. "We need every American who loves democracy to join us in 2024 to make sure that we make our democracy stronger, not weaker and jeopardized."

Later on Tuesday, Biden said: "We cannot let him win."

Trump tries to go after Biden on democracy

Trump devoted most of a speech in Cedar Rapids, Iowa, on Saturday to looking ahead to the general election -- potentially against Biden -- while trying to reverse Biden's democracy attacks.

His speech, focused on "saving democracy," started with him spouting false information about the 2020 presidential election, including wrongly claiming that Democrats "rigged" and "stole" it.

The 2020 results have been certified by a slew of local officials from both political parties and all claims of widespread fraud failed in court.

"Joe Biden is not the defender of American democracy. Joe Biden is the destroyer of American democracy and it's -- it's him and his people," Trump contended. "They're the wreckers of the American dream. The American dream is dead with them in office. It's sad."

Trump, who has also called himself a "very proud election denier," sought to link Biden to the populism that has been a key part of his campaign pitch since 2015.

"For decades, you watched as a corrupt political class in our nation's capital looted your money, trampled on your dignity and pushed their radical agenda into every aspect of your lives. You know it very well. But in 2016, you voted to stand up to those liars, losers, crooks and creeps and you elected an outsider as your president and it was about America first," he said.

"This campaign is a righteous crusade to liberate our republic from Biden," he went on to say.

ABC News' Soo Rin Kim and Kendall Ross contributed to this report.