The decision by Nebraska Republican Sen. Chuck Hagel to not seek re-election next year magnifies the defensive position of Senate Republicans in the 2007-08 campaign cycle.

Hagel announced his retirement Monday, just 10 days after Virginia Republican Sen. John W. Warner said he would not seek re-election. A third Republican, Colorado Sen. Wayne Allard, said earlier this year that he also will retire at the end of 2008. Democrats have no senators planning to retire.

Nebraska, Virginia and Colorado are among the 22 states that Republicans will be defending in Senate contests next year, compared with just 12 for the Democrats, who are strongly favored to retain or expand their 51-49 majority in the Senate.

While Nebraska typically votes heavily Republican in presidential elections — President Bush took 66 percent of the vote there in 2004, his fourth-highest total in the nation — Democrats have historically performed well in Senate races in that state and expect to wage a highly competitive campaign for Hagel’s seat.