Kris W. Kobach, a Republican who rose to national prominence among conservatives through his hard-line positions on immigration and voting rights, announced on Monday that he would seek the Kansas Senate seat held by Pat Roberts, who is retiring.
Standing in front of a campaign banner that read, “Build the Wall,” Mr. Kobach, who recently championed an effort to build a border wall with private funding, said he would work in the Senate to lead the charge for President Trump — though a statement from a Republican organization suggested Mr. Kobach would not receive official party support.
“This is a time I believe we all have to heed what J.F.K. said and ask what we can do for our country because our country needs us. And that’s what brings me here today,” he said.
Speaking before a group of about 50 supporters and reporters, Mr. Kobach vowed that, if elected, he would work to ensure that the United States does not provide any health benefits to immigrants in the country unlawfully.
Mr. Kobach, a former Kansas secretary of state, was the Republican nominee for governor there last year; he lost to Laura Kelly, the Democratic candidate, by 5 percentage points in a state that has long been reliably Republican. Since that stinging loss, members of the party had expressed concern that Kansas voters were seeking a more moderate platform than the one championed by Mr. Kobach.
Noting that loss, the National Republican Senate Committee, which raises money for Republican Senate candidates, issued a statement indicating Mr. Kobach would not receive its endorsement.
“Just last year Kris Kobach ran and lost to a Democrat,” said the statement, issued by an N.R.S.C. spokesperson. “Now, he wants to do the same and simultaneously put President Trump’s presidency and Senate Majority at risk. We know Kansans won’t let that happen and we look forward to watching the Republican candidate they do choose win next fall.”
Mr. Kobach, 53, a lawyer with degrees from Harvard, Yale and Oxford, was the latest in a large field of candidates lining up to replace Mr. Roberts, 83, who is in his fourth term.
Republicans who have already announced candidacies include Dave Lindstrom, a former lineman for the Kansas City Chiefs, and Jake LaTurner, the state treasurer.
Mr. Kobach’s candidacy could rekindle efforts by Republicans to recruit Secretary of State Mike Pompeo, a former representative from Kansas, as a candidate for the Senate seat. Mr. Pompeo said in February that he had ruled out a run despite encouragement from party leaders.
Others believed to be considering the race include Roger Marshall, an obstetrician who serves in the House of Representatives; former Gov. Jeff Colyer; and Alan Cobb, president of the state Chamber of Commerce.
Mr. Cobb, a former adviser to President Trump’s 2016 campaign, confirmed on Monday that he is mulling a run. “I think I’d bring a lot of things to the table that the others don’t,” he said.
Democrats who have already entered the race include Barry Grissom, a former United States attorney, and former Representative Nancy Boyda.
Mr. Trump had endorsed Mr. Kobach in the governor’s race last year, but some Kansas Republicans said they believed such an endorsement was unlikely in 2020, noting that Mr. Kobach had failed to capitalize on fund-raising and Mr. Trump’s endorsement.
Responding to Mr. Kobach’s announcement, the Senate Leadership Fund, a Super Pac aligned with allies of the Senate majority leader, Mitch McConnell, posted a statement on its website.
“Kansas Republicans deserve a nominee who can win,” the statement said, adding that, “After last year’s gubernatorial result, it is imperative Republicans put our best foot forward in Kansas.”
Before Mr. Kobach’s announcement, a statement of candidacy was filed in his name with the Federal Election Commission, although it initially misspelled his first name as “Chris.”