Until recently, Biden would only call Trump "the other guy" or make the sign of the cross when discussing him, as the veteran Democrat tried to focus on his own economic policies.
"He's like Voldemort now: His name shall not be mentioned," talk show host Conan O'Brien joked in a recent interview with Biden, comparing Trump to the unnameable villain from the Harry Potter novels.
Keen to avoid fueling Trump's claims of a political witch hunt, Biden also steadfastly avoided talking about the multiple criminal cases against the former president and reality TV star.
The result was that Trump could land his punches undefended: relentlessly mocking Biden's age (even though he's only four years younger) and mental capacity, and falsely accusing Biden of running a crime family.
But as Trump edges ahead in the polls and Democrats call for Biden to land some punches, the incumbent has in recent weeks taken a notably more confrontational approach to his rival.
- 'Doubling down' -
From private campaign events to big on-camera speeches, Biden now regularly criticizes Trump by name and even compares him to dictators of the past.
"It's self-evident. You saw it all," Biden said in the battleground state of Milwaukee on Wednesday after a reporter asked whether he considered Trump an "insurrectionist" after the explosive Colorado ruling.
In a seismic decision, the court blocked Trump from appearing on the Colorado primary ballot over his involvement in the January 6, 2021 assault on the Capitol by supporters trying to overturn his 2020 election loss to Biden.
It is the first of a number of legal actions across the country to successfully invoke the US Constitution's 14th Amendment, which bars from office anyone sworn to protect the country who later engages in insurrection.
"Now whether the 14th Amendment applies, I'll let the court make that decision. But he certainly supported an insurrection. No question about it, none, zero," said Biden.
"And he seems to be doubling down on about everything."
Biden, meanwhile, has been doubling down on his Trump attacks.
- 'Greatest threat' -
Aides say the plan until now had been to focus on Biden's achievements -- including a growing economy and low unemployment, even if many people are still feeling the pinch of inflation -- and not get drawn into catfights that might favor Trump with less than a year to go until Election Day.
But worries have grown in the White House as Biden's poll numbers have slumped.
The Washington Post reported this week that Biden had "stern words" for top aides about his "unacceptably low" polling just before the Thanksgiving holiday in November, as his economic message fell flat.
Biden has also been under pressure to hit back directly against Republican lawmakers who have just opened an impeachment inquiry into whether Biden profited from his troubled son Hunter's business dealings.
The result has been a marked shift in gears.
Biden now regularly reminds voters that he was the man who beat Trump in 2020 -- and sells himself, once again, as the only way to save the soul of America.
Biden recently pounced when Trump said that he would be a "dictator" his first day if reelected, and he has accused Trump of echoing the language of Nazi Germany when speaking about immigrants and his opponents.
In a speech in Milwaukee on Wednesday to promote his "Bidenomics" policies, the president went off subject to lambast Trump as "the guy who thinks we're polluting the blood of America" with migration.
Behind the closed doors of a campaign event at a donor's home in the smart suburbs of Washington on Tuesday, Biden was even more explicit.
"The greatest threat Trump poses is to our democracy," he said. "Because if we lose, we lose everything."
