Chad Taylor, the district attorney for Shawnee County, gave no reason for his decision, which was announced Wednesday. His departure leaves Greg Orman, an Olathe businessman running as an independent, as the only serious challenger to Sen. Pat Roberts.
Recent polls have suggested that Roberts, seeking a fourth term, is vulnerable but that Orman and Taylor were splitting the opposition vote.
Secretary of State Kris Kobach, a Republican, said Kansas law does not allow Taylor to take his name off the ballot. To do so, he would have to prove he is "incapable" of serving in the Senate if he wins.
"The law is the law," Kobach said.
Sen. Claire McCaskill, a Democrat from neighboring Missouri, confirmed that she discussed the race with Taylor before his announcement.
"I was happy to visit with Chad Taylor as he wrestled with a difficult decision," she said in a statement Thursday. "He is a respected prosecutor, a great guy and I wish him well."
The Democrats currently hold 53 seats in the Senate and the Republicans 45, while two independents, Angus King of Maine and Bernie Sanders of Vermont, caucus with the Democrats.
The Kansas Republican Party asked the secretary of state to determine if Taylor's name can be taken off the ballot so close to the election. Kelly Arnold, the party's state head, accused the Democrats of "backroom deals."
"It sure looks like the Kansas Democrat party bosses could not care less about what their own voters think and forced Taylor out," Arnold said Thursday. "The people of Kansas deserve to know what is going on behind the closed doors of the Democrat power brokers."