By Nandita Bose and Trevor Hunnicutt

WESTON, Massachusetts (Reuters) -U.S. President Joe Biden said on Tuesday he may have skipped a re-election bid if he were not facing Donald Trump, adding that the Republican posed a unique threat to the country.

“If Trump wasn’t running, I’m not sure I’d be running,” Biden said at a fundraising event for his 2024 campaign outside of Boston. “We cannot let him win.”

Biden’s remarks come as even staunch Democratic voters have expressed concerns about the president’s age. He turned 81 years old last month.

The president’s aides increasingly think Trump will cement his status as a frontrunner for the Republican presidential nomination in coming weeks, according to two of those Democrats.

During his 2020 presidential campaign, Biden often mentioned that his decision to run was due in part to then-President Trump’s handling of issues, including a 2017 white nationalist rally in Charlottesville, Virginia.

Now, Biden’s campaign is again positioning Trump as a danger to democracy itself.

Trump, who faces criminal charges over his efforts to reverse his 2020 election loss, has painted Biden as a dangerous autocrat.

Biden announced his re-election bid in April, after coming to the private belief that neither Vice President Kamala Harris nor any other Democratic hopeful could beat Trump in next year’s general election, according to a former White House official.

Biden has repeatedly made comments about Trump during a fundraising blitz that started on Tuesday in Boston and is set to include at least nine events before the end of the month.

Recent polling has shown the Republican frontrunner leading Biden in hypothetical matchups in key swing states and on the national level.

“I don’t think anyone doubts our democracy is at risk again,” Biden said earlier on Tuesday.

(Reporting by Nandita Bose in Weston, Massachusetts Writing by Trevor HunnicuttEditing by Chris Reese and Matthew Lewis)