Joe Biden swiftly seized on Donald Trump’s suggestion that he would cut “entitlements” like social security and Medicare if re-elected, saying: “Not on my watch.”

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That was in a presidential tweet. In a campaign speech in New Hampshire on Monday, Biden had elaborated.

“Many of my Republican friends want to put social security and Medicare back on the chopping block again,” he said. “If anyone tries to cut social security or Medicare or raise the retirement age again, I will stop them.

“We have two ways to go at social security and Medicare. Republicans will cut social security and Medicare to give us more tax cuts for the wealthy. Even this morning, Donald Trump said cuts to social security and Medicare are on the table again …

“I’m never going to allow that to happen. I won’t cut social security. I won’t cut Medicare. Instead of … giving tax breaks to the wealthy, I will protect and strengthen social security and Medicare and make the wealthy begin to pay their fair share.”

Most US adults oppose cuts to social security and Medicare. Trump campaigned for the Republican nomination promising to protect both, an effective card to play against rivals including Nikki Haley who said cuts should be considered.

But on Monday, Trump told CNBC: “So first of all, there is a lot you can do in terms of entitlements, in terms of cutting. And in terms of also the theft and the bad management of entitlements – tremendous bad management of entitlements – there’s tremendous amounts of things and numbers of things you can do.”

He also said Democrats would “end up weakening social security because the country is weak. I mean, take a look at outside of the stock market …… We’re going through hell. People are going through hell.”

One Trump spokesperson said he had only been referring to “waste and fraud”, while another said he had “delivered on his promise to protect social security and Medicare in his first term” and would “strongly protect” the programmes if returned to power.

For Biden, the White House deputy press secretary Andrew Bates said: “As the president just warned in his State of the Union address, Republican officials plan to cut Medicare and social security.”

Biden gave the State of the Union address last Thursday. Last year, in the same speech, he was widely praised for drawing Republicans into vocal denial that they wanted to cut social security and Medicare.

On Monday, Bates added that “today, in his budget, President Biden honours his ironclad commitment by firmly opposing benefit cuts to Medicare and social security”.

Any presidential budget is more a campaign statement than a concrete plan for action, given that Congress holds the power of the purse.

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Nonetheless, every one of Trump’s presidential budget proposals included cuts to social security and Medicare. Project 2025, a high-profile plan for a Republican transition should Trump win election this year, also raises the prospect.

Eight months out from election day, the battle lines over social security and Medicare seem well set.

On Monday, Martin O’Malley, the former Maryland governor who is now social security commissioner, said: “The administration supports efforts to improve social security benefits … especially for those who face the greatest challenges making ends meet.”

Biden’s campaign, meanwhile, tweeted out a montage – “of Trump calling to cut social security and Medicare (including this morning)”.