Secretary of State Mike Pompeo on Monday told Mitch McConnell, the Senate majority leader, that he does not plan to run for Senate in 2020, most likely ending Republicans’ hopes of securing a potentially dominant candidate for the open seat in his home state of Kansas, according to four people briefed on the meeting.
Mr. Pompeo, a former congressman from the Wichita area, has quietly explored a campaign for months. But in the aftermath of the military operation last week that killed Maj. Gen. Qassim Suleimani of Iran, Mr. Pompeo has told senior party officials that he is ruling out becoming a candidate, according to several people who have spoken with him directly.
Mr. Pompeo still has time to change his mind. The filing deadline for the primary is not until June. However, administration officials who have spoken with him in recent days said he seemed adamant about not entering the race.
Without Mr. Pompeo in the race, Republicans face an unsettled primary that includes at least one candidate, Kris Kobach, whom party leaders fear could imperil their hold on a crucial open Senate seat. Mr. Kobach, who ran unsuccessfully for governor in 2018, is popular with hard-right primary voters but widely disliked among moderate and independent voters in Kansas.
Several other candidates are competing for the Republican nomination, including Representative Roger Marshall. National Democrats have rallied behind Barbara Bollier, a state senator who left the G.O.P. to become a Democrat a little more than a year ago, as their preferred candidate.
Mr. Pompeo’s decision is a setback for Republicans working to retain their Senate majority in the November elections. Mr. McConnell aggressively courted him for months, and also deployed a number of his lieutenants to make the case that the secretary of state should return to Kansas, which he represented in the House until he joined the Trump administration.
Republicans hold a relatively narrow, 53-seat majority in the Senate, and they appear to be on defense in nearly all of the competitive races this year, including in blue states such as Colorado and Maine and Republican-leaning swing states like North Carolina, Arizona, Georgia and Iowa. Democrats have only one incumbent senator who is clearly vulnerable to defeat, Doug Jones of Alabama.
But both parties have been trying to stretch the Senate battlefield, hoping to increase their odds of an outright majority through an upset win or two. For Democrats, Kansas is among those long-shot targets, along with Texas, Kentucky and South Carolina.
The Republican majority in the Senate has been among the most important political assets of Donald J. Trump’s presidency, allowing him — even after Democrats seized the House of Representatives in 2018 — to appoint scores of conservative judges and, so far, to thwart the impeachment process initiated in the House.
Even before the Suleimani strike, that impeachment process had emerged as a political complication for Mr. Pompeo, because of his proximity to Mr. Trump’s efforts to spur the Ukrainian government to investigate former Vice President Joseph R. Biden Jr., and to the internal Trump administration debate about whether to withhold military aid from the Eastern European ally.
Still, Republicans held out hope that Mr. Pompeo would return to electoral politics in Kansas, and for months he encouraged that expectation.
While the state has not elected a Democrat to the Senate since 1932, Mr. McConnell and other top Republican officials are deeply worried that they could lose the seat should Mr. Kobach, the controversial former Kansas secretary of state, emerge as their nominee. An immigration hard-liner with a history of inflammatory comments, Mr. Kobach lost the governor’s race there last year.
Internal Republican polling suggested that Mr. Pompeo would have easily captured the nomination for the seat that is being vacated by Senator Pat Roberts, a longtime member of the chamber who is retiring. And for months, it seemed like Mr. Pompeo would go forward with a run as he found reasons to return to Kansas and conspicuously showed up at events in Washington that were connected to his home state.
He even appeared to win something of a blessing from Mr. Trump after Mr. McConnell and other congressional Republicans privately emphasized to him the importance of retaining the seat.
The fate of the Senate in the 2020 election could determine whether Mr. Trump would be able to accomplish anything in his second term, should he win another four years in office, or whether an incoming Democratic president would be able to enact significant legislation and appoint judges without worrying about a wall of Republican opposition in the upper chamber.