In the final days of a notably nasty Republican Senate primary race in Ohio, a conservative candidate is getting an unusual boost from the Democratic Party.

As my colleague Michael Bender reported last week, a Democratic group is spending $2.7 million on an ad highlighting the conservative credentials of Bernie Moreno, the Cleveland-area businessman backed by former President Donald Trump.

The ad says Moreno is a “MAGA Republican” who is “too conservative for Ohio,” describing him in terms that are likely to make him more appealing to conservative Ohio primary voters. Party strategists believe Moreno will be an easier opponent for the incumbent Democratic senator, Sherrod Brown, in the general election.

Meddling in Republican primaries has become a regular tactic for Democrats in recent years. But it has become more ideologically complicated as President Biden campaigns on the need to defend democracy from Trump-aligned Republicans like Moreno.

In his Senate bid, Moreno has embraced Trump’s falsehoods about the 2020 election, repeating the former president’s discredited charges of a “stolen” election and calling those prosecuted for storming the Capitol “political prisoners.”

The risk of such primary meddling is that Democrats could undercut Mr. Biden’s message about the threat to democracy and further erode trust in elections. Not to mention, of course, the chance that Moreno could win in November.