The strategy has already paid off in West Virginia.
One of the first calls this year from Senator Steve Daines, a Montana Republican overseeing his party’s Senate races, was to Mr. Justice in West Virginia, believing that the popular governor’s presence in the race would help persuade Mr. Manchin to retire.
The second part of Mr. Daines’s strategy in West Virginia was persistently lobbying to secure a Trump endorsement for Mr. Justice, with the aim of not just forcing out Mr. Manchin but also the hope that it would convince him to run for president as an independent. Mr. Trump endorsed Mr. Justice last month.
Mr. Manchin, meanwhile, furthered speculation of a potential presidential bid by saying Thursday he planned to gauge “interest in creating a movement to mobilize the middle and bring Americans together.”
In 2018, Democrats and Republicans combined spent about $53 million on the West Virginia Senate race. With no competitive race there in 2024, both parties will have tens of millions of dollars to spend on a second tier of battleground races. Last year, candidates, parties and outside groups spent more than $1.3 billion on 36 Senate races, including $737 million in just five states — Arizona, Nevada, Ohio, Pennsylvania, and Wisconsin — that are also on the ballot again next year.
“I think Wisconsin and Michigan are about to get a bunch of Republican money they weren’t going to get otherwise,” said Brad Todd, a Republican strategist who has worked on Senate races.