U.S. Agriculture Secretary Mike Johanns will resign his Cabinet post to run for the U.S. Senate in Nebraska, a state Republican official said Wednesday.

The official, speaking on condition of anonymity, said Johanns was planning to announce his bid to replace outgoing Republican Chuck Hagel as early as Monday.

A run by Johanns would make him the fourth declared Republican candidate. The others were Nebraska Attorney General Jon Bruning, Schuyler businessman Pat Flynn and former U.S. Rep. and Omaha mayor Hal Daub.

Johanns' primary spokeswoman, Terri Teuber, would not say whether Johanns planned to run, but said he would make a decision soon.

"Secretary Johanns understands that this is a decision he needs to make in the immediate future and he intends to do so," Teuber said in an e-mail Wednesday. She did not respond a direct question of whether Johanns had already quit.

Johanns would have to resign before announcing his candidacy under the Hatch Act, which prohibits federal employees from being candidates for public office in partisan elections.

White House spokesman Alex Conant said President Bush would support Johanns no matter what he decides.

"Secretary Johanns is an outstanding secretary of agriculture, and the president will support whatever decision he makes," Conant said.

In resigning, Johanns would leave his post before Congress passes a new farm bill, a goal Johanns has said he wanted to reach by the end of the year.

The politically popular piece of legislation gives billions in aid to farmers and pays for the nation's nutrition programs, but expires at the end of this month.

"We're at a critical juncture for agriculture with the reauthorization of the farm bill still not completed," said Sen. Ben Nelson, D-Neb. "Right now, it's a tough time for farmers and ranchers. These issues shouldn't take a back seat to elections and politics."

Johanns would be the most high-profile candidate for the Republicans in Nebraska, who are fighting to keep the Democrats from widening their national majority.

Democrats control the 100-member Senate with 49 seats plus two independents who usually vote Democratic. Republicans must defend 22 seats next year, compared with 12 for Democrats.

Virginia Sen. John Warner and Colorado Sen. Wayne Allard both intend to retire, and incumbents in New Hampshire, Oregon, Minnesota and Maine face extra-tough races.

In addition, Alaska Sen. Ted Stevens faces a federal corruption investigation, and Idaho Sen. Larry Craig is struggling after his arrest in an airport men's room sex sting.

Democrats are hoping that Bob Kerrey, a former Nebraska governor and U.S. senator, will return to Nebraska politics. Kerrey said Wednesday he is still considering it, as are Omaha Mayor Mike Fahey and Scott Kleeb, who lost the 3rd District House contest last year.

Kerrey said Johanns' candidacy would not affect his decision.

"The scales in my case are first personal to my family — when I ran before I didn't have a wife," Kerrey said. "I have a wife and a small child, and this would be very disruptive to our lives."

Kerrey is president of New School University in New York. The 64-year-old served as governor of Nebraska, 1983-1987, and in the Senate from 1997 to 2001. He married Sarah Paley in February 2001 and their first son, Henry, was born later that year.

Republicans have said that if Kerrey were to enter the race, his connections and popularity among some in the state would make him tough to beat.

Kerrey said Johanns would make a great candidate for the Republicans along with Daub and Bruning.

"You've clearly got three highly qualified candidates and oftentimes that's not the case," Kerrey said. "It's a good thing that people as qualified as Mike Johanns are willing to run."

Johanns, 57, was mayor of Lincoln from 1991 to 1998, when he was elected governor. He won re-election to a second term in 2002.

Johanns has so far refused to say publicly whether he is interested in Hagel's seat.

But on Saturday, as the state GOP dedicated its headquarters to the former governor and his wife, Stephanie, Johanns said he and his family will end up back in Nebraska when his Cabinet job is done.

He said he and his wife "miss Nebraska every day. Our home is Nebraska. We'll be back."

At the same gathering in Lincoln, Hagel said Johanns has what a candidate needs: "Mike has been privileged ... to be able to have the trust of the people, and that's what elections are about."