While Republicans struggle to get President Trump to say he will support their party’s nominee — no matter who it is — and not launch a third-party run if he loses the 2024 presidential nomination, threats of a third-party candidate are also spooking Democrats, who suspect they be hurt most by such a move.

In late February, the Republican National Committee chairwoman, Ronna McDaniel, announced the group would require candidates to pledge to support the party’s nominee if they want to participate in the GOP primary and any debates it may organize.

The move was aimed at corralling Mr. Trump, who has repeatedly refused to say he would support the eventual nominee and suggested he might try running as a third-party candidate.

Now, Democrats are also beginning to worry that they could suffer should an independent candidate launch a third-party bid. Support for a third-party Democratic challenger could come from a new group named No Labels, which is teasing a so-called unity ticket as an “insurance policy” against a 2024 rematch between Presidents Biden and Trump. 

The group told the New York Times that it had raised $46 million of its $70 million goal so far, and that more than 400 canvassers are actively collecting signatures to help it get on ballots. In Florida, North Carolina, and Nevada, the group is already seeking ballot access, and No Labels has gained ballot access in Colorado and Arizona.

The group has a nominating convention scheduled for April 2024 in Dallas, and it has stressed that it is courting political moderates, both Democratic and Republican.

Enter Larry Hogan, a popular former Republican governor of Maryland, a deep blue state, who has said specifically that he would not seek the Republican nomination. “It was a tough decision, but I’ve decided that I will not be a candidate for the Republican nomination for president,” Mr. Hogan told CBS News on Monday.

His choice of wording has spurred rampant speculation that Mr. Hogan, a vocal critic of both Mr. Trump  and Mr. Biden, may enter the race as a third-party candidate.

While Mr. Hogan may be a Republican, he — or a candidate in his mold — would likely help instead of hinder the Republican nominee, according to an associate editor at Sabato’s Crystal Ball, Miles Coleman.

“The voters Hogan may draw would theoretically be your wealthier, more college-educated voters who may have been Republicans before Trump,” he told the Sun.

According to Mr. Coleman, these voters were “key” to Mr. Biden’s win in competitive states like Wisconsin and Arizona, and a third-party candidate siphoning away voters in these states would cause more trouble for the Democratic candidate than the Republican candidate.

Mr. Hogan’s decision, Mr. Coleman said, was likely based more on opposing Mr. Trump’s candidacy than in advancing his own third-party bid. Mr. Coleman posed the question: “After a savvy calculation like that, how could Hogan turn around and float something as silly as a third-party run?”

“Hogan’s decision not to run for the GOP nomination, though it went against expectations, was actually a good one from a strategic perspective: the fewer candidates in the ring, the easier it would be for someone to consolidate the anti-Trump vote,” Mr. Coleman said.

This appears to have been Mr. Hogan’s analysis of the situation as well, as he has stressed the importance of not nominating Mr. Trump again, and telling CBS News that he is pushing for the party to “return to real Republicanism.”

While Mr. Coleman doesn’t think that Mr. Hogan will be running as a third-party candidate, that doesn’t mean that the No Labels group won’t nominate someone who could damage the Democrats’ 2024 chances.

In a map released by No Labels, the group claims that an “American Unity” ticket would carry 279 electoral votes with the right candidates, including Florida, Texas, and Mr. Biden’s home state of Delaware. Only 270 electoral votes are needed to win the presidency.

Although that map is optimistic, the group would need to win the votes of about 1 percent of Mr. Biden’s supporters in swing states like Arizona, Georgia, Pennsylvania, and Wisconsin to change the results.