Biden announces 2024 re-election bid

"Every generation has a moment where they have had to stand up for democracy. To stand up for their fundamental freedoms," Biden wrote on Twitter, along with a video.

"I believe this is ours. That’s why I’m running for re-election as president of the United States. Join us. Let's finish the job."

After a series of big legislative wins and momentous foreign policy struggles in his first two years in office, Biden has no real challenger from within the Democratic Party.

But in a campaign that may result in a rematch of the 2020 election against Donald Trump, he is expected to face constant and fierce scrutiny over his age.

The veteran Democrat would be 86 by the end of a second term. Even if a medical exam in February found him "fit" to execute the duties of the presidency, many including in his own voter base believe he is too old.

An NBC News poll released over the weekend found that 70 percent of Americans, including 51 percent of Democrats, believe he should not run.

Sixty-nine percent of all respondents who said he shouldn't run cited concerns over his age as a major or minor reason.

Biden likes to answer those concerns by saying, "watch me" -- meaning that voters should focus on his policy wins at home and his marshaling of an unprecedented Western alliance to help Ukraine defend itself against Russia's invasion.

Over the next year and a half, Biden will have all the advantages of incumbency, backed by a united party, while Republicans are only just starting a messy primary season.

Trump, despite becoming the first former or serving president to be criminally indicted -- and facing probes into his attempt to overturn his loss to Biden in the 2020 election -- is the overwhelming Republican frontrunner.

On Monday, Trump was quick to pitch in his own criticism of the man who defeated him last time around.

"With such a calamitous and failed presidency, it is almost inconceivable that Biden would even think of running for reelection," he said in a statement.

The most likely Republican challenger to the 76-year-old Trump, Florida Governor Ron DeSantis, presents a similarly right-wing figure, though starkly younger at 44.