That will open up a tough seat for Republicans to defend. They have had a bad streak in Missouri statewide races, beginning with Sen. Jim Talent’s loss in 2006 and Rep. Kenny Hulshof’s (R.) disastrous run for governor last year. (John McCain did narrowly carry the state, however.)

The first Senate retirement announcement came earlier, from Sam Brownback (R.) of neighboring Kansas. He will be leaving Washington next year to run for governor.

UPDATE: A reader reminds me that in addition to Bond and Brownback, Mel Martinez of Florida called it quits earlier. That makes three Republicans retiring from the Senate in 2010 so far, at least two of whose seats will be quite difficult to defend.