Sanders has been a reliable Biden ally on most of the president’s domestic agenda.

But he has become a thorn in the administration’s side over the mounting death toll in Israel’s response to the October 7 Hamas attacks.

That assault seven months ago resulted in the deaths of more than 1,170 people, mostly civilians, according to an Agence France-Presse tally of Israeli official figures.

Israel has responded with a protracted military campaign that has so far killed at least 34,735 Palestinians, according to the Gaza health ministry on Monday. The ministry said in February that 70 per cent of those killed by Israel were women and children.

“Israel had the absolute right to defend itself against this terrorist attack, but it did not and does not have the right to go to war against the entire Palestinian people, which is exactly what it is doing,” Sanders said in his announcement.

“In my view, US taxpayers should not be providing billions more to the extremist Netanyahu government to continue its devastating war against the Palestinian people,” he said.

Sanders, who suffered a heart attack during his 2020 presidential campaign, is the second-oldest senator, behind 90-year-old Republican Chuck Grassley.

He is almost certain to be reelected by his liberal northeastern state, meaning he would be 89 at the end of the coming term.

“Will the United States continue to even function as a democracy, or will we move to an authoritarian form of government?” he said.

A fringe figure for much of his career, Sanders saw his political stock rise as he campaigned twice to represent the Democrats in presidential elections on a platform of universal healthcare, fairer housing and low-priced universities.

And his 2020 bid was cut short as the centrist Democratic establishment pulled together in a coordinated effort to support Biden.

Additional reporting by Associated Press