Updated

Agriculture Secretary Mike Johanns announced his intent on Thursday to hop into what is amounting to be one of the more hotly contested Senate races in 2008, the battle for an opening in Nebraska.

Johanns, in an announcement with President Bush from the White House Rose Garden, called his chance at the Senate "the opportunity of a lifetime" and started with a campaign appeal directed at fellow Nebraskans. Johanns is resigning his current post, and deputy secretary Chuck Conner will serve as acting secretary.

Johanns will be running for the seat filled by Republican Sen. Chuck Hagel. Former Sen. Bob Kerrey, D-Neb., also was expected to run.

"In a sense I've brought Nebraskans with me to every cabinet meeting, every hearing on Capitol Hill, every negotiating session in faraway countries. I often thought of the wisdom they conveyed to me during my years as governor (of Nebraska). I often thought of their decency, and I did my best to remain true to the common-sense, practical approach of Nebraskans," Johanns said.

"To me, it's a dream come true for a farmer's son," Johanns added.

Bush praised Johanns' work on renewable energy and in other areas and said he believed Johanns would be a good addition to Congress.

In a recent meeting with Johanns, Bush asked him about the rumors that Johanns was thinking of seeking the Senate seat. Bush said Johanns said it was true, and he convinced him it was something he really wanted to do.

"My answer to Mike was, you know, 'I support you, and I encourage you to follow your heart.' If it's Mike's decision, and Nebraska's choice, he would make an outstanding member of the United States Senate. There is no doubt in my mind," Bush said.

Johanns' entry was likely to be welcomed news to fellow Republicans, who are facing tough battles next year and already have lost one of their most senior and respected members to retirement, Virginia Sen. John Warner. That race also was shaping up to be a bruising battle, with former Gov. Mark Warner, a Democrat, last week throwing his hat into the ring and several Republican candidates champing at the bit.

Bush also was working to fill another key vacancy, that left by former Attorney General Alberto Gonzales. Bush has nominated retired federal judge Michael Mukasey to replace him.

Inside the White House, Bush has seen the departures recently of longtime advisers Karl Rove and Dan Bartlett. Press secretary Tony Snow's last day was last Friday.

The Associated Press contributed to this report.