Updated: Feb. 15, 2:45 p.m.

Sen. Evan Bayh (D-Ind.) announced Monday that he will not seek re-election this year, a stunning eleventh-hour development that gives Republicans a prime opportunity to capture a seat they had no expectation of winning at the beginning of the election cycle.
 

“After all these years, my passion for service to my fellow citizens is undiminished, but my desire to do so by serving in Congress has waned,” Bayh said at a news conference Monday afternoon in Indianapolis.
 

Bayh, a popular former governor and a son of former Sen. Birch Bayh (D-Ind.), was easily elected to the Senate in 1998 and overwhelmingly re-elected in 2004. But Republicans were gearing up to seriously challenge him this year. Former Sen. Dan Coats recently announced his candidacy and a few other Republicans have been campaigning for months.
 

Nonetheless, Bayh insisted that his decision was not influenced by political calculations. Bayh had completed his candidacy paperwork, was polling ahead of Coats and had $13 million in his campaign account.
 

“My decision was not motivated by political concern,” Bayh said. “Even in the current challenging environment, I am confident in my prospects for re-election. Running for the sake of winning an election, just to remain in public office, is not good enough.”
 

Bayh said that there has been “too much partisanship” in the Senate and decried efforts to scuttle a bipartisan jobs bill as well as a plan to create a bipartisan commission to recommend ways to control the nation’s debt.
 

Bayh’s decision puts Democrats in a bind because it comes just one day before Senate candidates must submit voter signatures to county election offices and four days before they must file for ballot access with state election officials in Indianapolis. To qualify for the May 4 primary ballot, a Senate candidate must collect 4,500 signatures of registered voters — including at least 500 in each of the state’s nine congressional districts.
 

Robert Dion, a political scientist at the University of Evansville, said that Bayh’s last-minute decision is “out of character for him.”
 

“He’s always been really methodical and calculated, not one who is given to these kinds of odd impulses,” he said.
 

With no Democrat likely to meet tomorrow’s deadline, party officials in Indianapolis will select a replacement candidate for Bayh. Indiana election law provides that “a candidate vacancy for United States Senator or a state office shall be filled by the state committee of the political party.”
 

Potential candidates for the nomination include Rep. Brad Ellsworth, who represents the southwestern 8th district, and Rep. Baron Hill, who represents the southeastern 9th district and lost a 1990 Senate race to Coats.
 

Ellsworth released a statement praising Bayh and expressing appreciation for “the support of those Hoosiers who have already encouraged me to run for Senator Bayh’s seat.”
 

“The next step will be taking a few days to talk to my wife and to folks in Indiana about where I can best serve our state,” Ellsworth said.
 

Hill spokeswoman Katie Moreau said that the Congressman is on a military trip overseas and unavailable for comment.
 

Coats is expected to qualify for the GOP primary along with former Rep. John Hostettler, state Sen. Marlin Stutzman and several lesser-known candidates.
 

CQ Politics presently rates the Indiana Senate race as Leans Democratic, though that rating will be changed now that Bayh is not running.