Democrats working to boost a Trump-endorsed Republican candidate for the US Senate in Ohio, as a way to boost their own progressive senator, should be careful, an aide to the Republican said, lest such efforts backfire and they lose a precious seat.

Such tactics have been used by Democrats before – when they support a more extreme Republican to be nominated out of a calculation that that candidate will then stand less chance of winning against a Democrat in a general election. Of course, such tactics could backfire and see extremists elected.

“Democrats constantly underestimate the America First movement at their own peril,” Reagan McCarthy, communications director to Bernie Moreno, told news outlets.

“They thought President Trump would be easy to beat in 2016 and then they got their clocks cleaned when he demolished Hillary Clinton. The same thing is going to happen to Sherrod Brown this year.”

Democrats control the US Senate by 51 seats to 49. Republicans have high hopes of retaking the chamber, with Ohio on their list of targets.

Brown, 71, is a major presence on the Democratic left, first elected to the Senate in 2006 when he beat Mike DeWine.

DeWine is now the Republican governor of a former battleground state that has trended right. In the last Senate election, the bestselling author JD Vance, a self-described “conservative knuckle-dragger”, took the other seat.

This year’s Republican Senate primary sees Moreno, a businessman also endorsed by Vance, facing Matt Dolan, a state senator backed by DeWine, and Frank LaRose, the secretary of state. Polling indicates a close race with Dolan leading. Election day is next Tuesday.

On Thursday, multiple outlets reported that Duty and Country, a group linked to the Democratic Senate Majority political action committee, was spending about $2.7m on an ad to run across the state, meant to boost Moreno among Republicans by calling him “ultraconservative”, “too conservative for Ohio” and “too aligned” with Trump.

“Donald Trump needs Bernie Moreno,” said the ad, titled Maga Fighter, in reference to Trump’s endorsement.

“Ohio doesn’t.”

The aim is to give Brown a better chance in the general election, against a candidate Democrats can portray as too extreme, linked to attacks on reproductive rights and democracy itself, key themes for Democratic campaigns from Joe Biden down.

Hannah Menchhoff, a spokesperson for the Senate Majority Pac, said: “When Ohio voters head to their polling place, they deserve to know the truth about Bernie Moreno – and the truth is that Moreno is a Maga extremist who embraced Donald Trump just like he embraced his policies to ban abortion nationwide and repeal” the Affordable Care Act.

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A similar Democratic effort recently succeeded in California. In an open primary for a US Senate seat, the former US House intelligence chair Adam Schiff successfully boosted the Republican Steve Garvey past other Democrats, making for what should be an easy victory in November.

Democrats in Pennsylvania – much more of a battleground than deep-blue California – have also enjoyed success with such tactics. In 2022, they played up the conservative credentials of Doug Mastriano, a Trump-endorsed candidate for governor with far-right links and views. The Democrat, Josh Shapiro, won the election with ease.

Reed Galen, a Republican operative turned co-founder of the Lincoln Project, an anti-Trump group, said that such tactics were “relatively common but not always sound.

“The risk of course is that you lose to the Trumpier guy. But it’s hardball – something Democrats aren’t typically known for,” he said.

Galen also pointed to Brown’s strength even when targeted by national Republicans, saying the senator had a “better than average” chance of re-election.

According to polling released on Wednesday by Emerson College Polling and the Hill, Brown is indeed well placed to retain his seat. In hypothetical match-ups, the Democrat led LaRose 39% to 33%, Moreno 39% to 34% and Dolan 37% to 34%.

“He’s an institution,” Galen said of Brown.