Democrats are salivating over the prospect of running against President Trump in 2024 and, after boosting weak midterm opponents in several GOP primaries this year, they’ll meddle in the Republican presidential race if state legislators fail to act.

As the Washington Post reported in September, “Democrats directly interfered in at least 13 primaries,” spending more than $53 million to secure their preferred general election opponents. Reuters reported Democrats intervened in two other primaries as well. Of these 15 “Manchurian Candidates,” so to speak, eight prevailed, and Democrats beat them all on Election Day.

“The results,” Reuters wrote, “could provide a blueprint for the 2024 presidential election,” and on Thursday, the Sun in an editorial asked if the Democratic majority leader will deploy the same method on the presidential level.

The editorial cited $3.2 million a PAC “aligned with” Senator Schumer spent championing one Republican Senate candidate alone, Donald Bolduc of New Hampshire, whose loss secured Democrats’ hold on the upper chamber of Congress.

Although even some Democrats expressed reservations about this strategy, it worked, and is now part of our political landscape. Because it’s legal, there’s little the RNC can do beyond retaliating by funding far-left candidates in Democratic primaries.  

Yet while Manchurian money is here to stay, there’s a more grassroots brand of meddling that can be stopped with some effort, at least in states where Republicans have enough legislative support to restructure who chooses delegates to national conventions.

According to OpenPrimaries.org, Republican primaries are closed to outsiders in 24 states, but 18 offer ballots to Democrats and the remaining eight allow unaffiliated or independent voters to have a say. With more partisans exploiting this loophole, some states have taken notice.

In August, according to Alabama Political Reporter, the state’s GOP executive committee “voted overwhelmingly in favor of recommending the restriction of primary participation to registered Republicans.” Now it’s up to the local legislature to act.

The Reporter added that party leaders “began discussing the matter after seeing Democrats sharing that they had participated in the Republican primary.” A county GOP chairman, Michael Hoyt, praised Governor DeSantis of Florida — a possible challenger to Mr. Trump in 2024 — for locking down that state’s primary to prevent subversion.

Because President Biden has declared his “intention” to run for a second term, his party’s contests will be a formality, barring the rise of a rival like Edward Kennedy, the senator who challenged the unpopular Democratic incumbent, President Carter, in 1980.

So, what will tens of millions of Democrats do with those primary ballots burning holes in their pockets? Well, they could use them to sow a little mischief in the GOP ranks by supporting the weakest horse on the track, which they are again convinced is Mr. Trump.

A former Democratic presidential candidate of Vermont, Senator Sanders, told the New York Times of Mr. Trump, “[A]s a politician who wants to see that no Republican is elected to the White House in 2024, from that perspective, his candidacy is probably a good thing.”

Other prominent party voices are excited about facing the former president, whose favorability rating is 19 points underwater, 39 percent to 58 percent, according to a 2022 CNN exit poll. Unlike the millions of dollars that Mr. Schumer and others spent boosting MAGA candidates, urging cross-over voting wouldn’t cost a dime.

Democrats faced a similar campaign in 2008 after President Obama had almost wrapped up the Democratic nomination. Republicans — urged on by my late boss Rush Limbaugh’s “Operation Chaos” — sought to keep the contest going through to the convention by crossing over to vote for Secretary Clinton.

Although Alabama has started the process of reform, Democrats in Arkansas, Georgia, Illinois, Indiana, Michigan, Minnesota, Mississippi, Missouri, Montana, North Carolina, Ohio, South Carolina, Tennessee, Texas, Vermont, Virginia, and Wisconsin can all have a say on who stands against Mr. Biden in 2024.

Mr. Trump may smoke the primary field without Democratic help and may even ultimately prevail in a rematch against Mr. Biden. Yet Republican leaders are on notice that the door is open to the opposition picking their party’s weakest candidates. Fail to close it, and they’ll have no one to blame if the electoral map in two years looks a lot like the one from 2020.