Republican Jeff Johnson replaced his campaign manager Friday, a late change in top leadership as he tries to mount a serious challenge to Gov. Mark Dayton with Election Day just six weeks off.

Johnson dropped the surprise news in a late afternoon news release. David Gaither, a former state senator from suburban Hennepin County and one-time chief of staff to former Gov. Tim Pawlenty, is taking over the campaign. He will replace Scot Crockett, who led Johnson’s successful effort to win the GOP gubernatorial primary in August.

“David Gaither’s skills and experience will be invaluable to my campaign as we make the final push to victory in the next six weeks,” Johnson said. Spokesman Jeff Bakken said it was Crockett’s decision to leave, wanting to return to his home in Virginia to be with his family.

Bakken said Crockett would continue to advise the campaign. A Minnesota native, Crockett had worked in the administration of former Gov. Arne Carlson and for U.S. Rep. John Kline before briefly leading a national Tea Party organization in 2012 and 2013.

With the election looming — absentee balloting started Friday — Johnson has struggled with an underfunded campaign and low name recognition.

A Star Tribune poll of 800 likely voters taken in early September found that a third of respondents had never heard of him, while another 40 percent had no opinion on the Hennepin County commissioner and former state lawmaker.

The same poll had Dayton leading Johnson, 45 percent to 33 percent, with 20 percent undecided.

Johnson’s campaign has not yet aired television commercials, while the DFL and third-party group Alliance for a Better Minnesota have aired ads critical of him. Johnson said earlier Friday he would be on the air by the end of September.

Campaign leadership shake-ups are not unusual; Mike McFadden, the Republican challenger to Sen. Al Franken, replaced his campaign manager after winning the Republican endorsement in May. But a leadership change at the top this close to the election is less common.

Like Johnson, Gaither hails from Plymouth. He served as a Republican state senator from 2002 to 2005, then resigned to serve as chief of staff under Pawlenty, generally seen as the top staff position in any gubernatorial administration. Gaither stayed in the job only a little over a year.

Most recently, Gaither ran for his old Senate seat in 2012 but lost to incumbent DFL Sen. Terri Bonoff. The Johnson campaign said Gaither would take a leave of absence from his job as executive director of the Minneapolis-based International Education Center.

Bakken said Friday night that Gaither was not available for an interview because he was on the job as an assistant coach for the Wayzata High School football team.

Bakken said Gaither had already been helping the Johnson campaign informally, including with the process that led to the selection of former state Rep. Bill Kuisle of rural Rochester as his running mate.

Johnson and Gaither represented the same Plymouth-area legislative district as fellow Republicans a decade ago.

“They served together and they are friends,” Bakken said.