A swing-state Republican senator denied threatening social security and Medicare, after Democrats accused him of putting them “on the chopping block”.

Ron Johnson, who entered Congress on the Tea Party wave of 2010, is up for re-election in Wisconsin. As they attempt to keep hold of the Senate, Democrats think they have a chance of winning the seat.

In an interview with The Regular Joe Show podcast, Johnson said social security and Medicare, crucial support programs for millions of older and disabled Americans and their dependents, should no longer be considered mandatory spending.

“If you qualify for the entitlement, you just get it no matter what the cost,” Johnson said. “And our problem in this country is that more than 70% of our federal budget, of our federal spending, is all mandatory spending. It’s on automatic pilot … you just don’t do proper oversight. You don’t get in there and fix the programs going bankrupt.”

He added: “What we ought to be doing is we ought to turn everything into discretionary spending so it’s all evaluated so that we can fix problems or fix programs that are broken, that are going to be going bankrupt. As long as things are on automatic pilot, we just continue to pile up debt.”

Democrats pounced. Chuck Schumer of New York, the Senate majority leader, referred to Donald Trump’s “Make America Great Again” slogan when he said: “They’re saying the quiet part out loud. Maga Republicans want to put social security and Medicare on the chopping block.”

A Johnson spokesperson said Schumer was “lying”.

The spokesperson said Johnson’s “point was that without fiscal discipline and oversight typically found with discretionary spending, Congress has allowed the guaranteed benefits for programs like social security and Medicare to be threatened.

“This must be addressed by Congress taking its responsibilities seriously to ensure that seniors don’t need to question whether the programs they depend on remain solvent.”

Social security payments average just over $1,600 a month.

Last year, Nancy Altman, president of Social Security Works, told the Guardian: “The nation is really facing a retirement income crisis, where too many people aren’t going to be able to retire and maintain savings to live on. It’s a very strong system, but its benefits are extremely low by virtually any way you measure them.”

Democrats see Republican threats to so-called “entitlements” – programs paid for by taxes and relied upon by vulnerable people – as a potent electoral issue. Polls show strong bipartisan support.

From Joe Biden to leaders in Congress, Democrats have seized on a plan published by Rick Scott of Florida, the chair of the Republican Senate campaign committee.

Scott proposed that all Americans should pay some income tax and that all federal laws should expire after five years if Congress does not renew them.

The senator insisted he was “not going to raise anybody’s taxes” – despite saying more people should pay tax. He also said Congress “needs to start being honest with the American public and tell them exactly what we’re going to do to make sure they continue to get their Medicare and their social security”.

But his own leader in the Senate, Mitch McConnell of Kentucky, said: “We will not have, as part of our agenda, a bill that raises taxes on half the American people and sunsets social security and Medicare within five years.”

Wisconsin will hold its primaries on Tuesday. Johnson is being challenged by the current lieutenant governor, Mandela Barnes.

Jessica Taylor of the Cook Political Report told Wisconsin Public Radio Johnson was national Democrats’ “No 1 incumbent … that they are targeting”.