WASHINGTON — National Republicans on Thursday moved to take control of the campaign of Senator Pat Roberts of Kansas by sending a longtime party strategist to the state to advise him, a day after his hopes for re-election and those of his party for taking control of the Senate were threatened by the attempted withdrawal of the Democrat in the race.

Also on Thursday, the Kansas secretary of state, Kris Kobach, a Republican, ruled that the Democratic nominee, Chad Taylor, could not withdraw his name from the ballot. Democratic officials said Mr. Taylor would file a legal challenge to the ruling.

The National Republican Senatorial Committee is sending Chris LaCivita, who has served as a political troubleshooter in past Republican campaigns, to counsel Mr. Roberts and help oversee his campaign. The committee will also seek to hire a local lawyer in any legal challenge against Mr. Taylor, who had tried to drop off the ballot on the last day candidates were allowed to do so.

Mr. LaCivita is expected to be in Kansas by this weekend.

Mr. Taylor’s attempt to drop out of the race elevated Greg Orman, a well-funded independent candidate whose television ads have been on the Kansas airwaves since July. With a Libertarian, Randall Batson, who may take votes from the right, also on the ballot, Mr. Roberts could be in jeopardy after having been thought to be a relatively safe incumbent.