From The Caucus Blog at NYTimes:

Agriculture Secretary Mike Johanns will resign his Cabinet seat tomorrow to begin preparing to run for a Senate seat from Nebraska, a senior administration official said tonight.

Mr. Johanns, a former Republican governor of Nebraska, is seeking to replace Republican Senator Chuck Hagel, who announced earlier this month that he will not seek re-election in 2008. Mr. Johanns told party officials and supporters that he will make his Senate announcement shortly after he leaves the administration.

Mr. Johanns would become the fourth Republican candidate in the field, which includes Nebraska Attorney General Jon Bruning. While Nebraska is one of the most Republican states in the nation, the vacancy created by Mr. Hagel is creating a potential opportunity for Democrats, who are attempting to recruit Bob Kerrey to run.