(AP Photo/Columbia Daily Tribune, Don Shrubshell)
Koster announced Wednesday that he was switching from the Republican to the Democratic Party.
Koster, of Harrisonville, said he has determined that he is more aligned with Democrats than Republicans on several issues, including stem-cell research, workers' rights, minimum wage and leaving intact Missouri's current judiciary system.
"Today, Republican moderates are all but extinct," Koster said at a news conference at the University of Missouri-Columbia campus, where he got his bachelor's and law degrees. "On so many of the critical issues of our day, it has been Democrats and not Republicans who have shared my beliefs and fought by my side."
He stopped short of declaring his intent to run for attorney general, but he has formed a campaign committee for an unspecified statewide office in 2008 and said he does not intend to seek re-election in the Senate.
"I don't want to get into election strategy today," he said, but added he was not concerned about losing or angering his donors.
Whether Democrats will embrace him is the question.
While some Democrats welcomed Koster to the party, others who are running for attorney general scoffed at the switch.
"For the past 30 years, I have stood up for Missouri families by championing Democratic values such as access to health care," candidate and Democratic Rep. Margaret Donnelly, of St. Louis, said in a written statement. "The voters of Missouri want stability and commitment, not political opportunism, in the next attorney general."
Koster defended his move as in line with his vision and priorities.
"Words are going to be used to frame this by a lot of different sectors," Koster said. "This has been a long struggle. This is what my gut tells me to do."
Critics also handed out fliers at the news conference from the Boone County Democratic Central Committee noting Koster's votes with the Republican majority on major issues such as cutting Medicaid benefits and requiring voters to show a photo identification to cast a ballot.
But Sen. Jeff Smith, D-St. Louis, said Democratic voters would give him a chance.
"There'll be some percentage of people who will clearly dismiss him out of hand. But I don't think that's the majority of primary voters," he said.
One of the state's most prominent Democrats, U.S. Sen. Claire McCaskill, also welcomed Koster.
Koster, 42, a lawyer and former prosecutor, has angered some in the Republican Party for his ardent support of stem-cell research. Some anti-abortion organizations believe a certain form of stem-cell research destroys life at its earliest stages.
Koster also said the party's -- and particularly Gov. Matt Blunt's -- criticism of the judiciary factored in his decision.
"My extraordinary disappointment over the vituperative that gets spit out of the administration toward the third branch of government has reached a boiling point," he said.
A Blunt spokesman responded that the governor represents the values of Missourians in Koster's district.
"The governor does not believe unelected judges should be allowed to raise taxes," spokesman Rich Chrismer said.
Jared Craighead, executive director of the Missouri Republican Party, said Koster's decision to switch parties was a strategic move, made because Koster thinks it will be easier to win the Democratic primary in the attorney general's race. The incumbent, Democrat Jay Nixon, plans to seek the governor's office.
"Chris is a personal friend of mine, but I fear he has blinded himself with his desire for higher office and the hollow promises of Jay Nixon's political machine, rather than keeping his commitment to the constituents who elected him to represent them," Craighead said in a written statement.
He also said Koster should resign from the Senate and run as a Democrat in a special election. State election officials were aware of no law requiring him to give up his office.
U.S. Rep. Sam Graves, R-Mo., who has known Koster for 15 years, said he was "surprised but not shocked" by the move. "Chris does what he has to do to get elected," Graves said.
In a letter late Tuesday, Koster told Republican Senate leaders that he was resigning his post as majority caucus chairman. The slot was listed as vacant by Wednesday morning.
Koster's campaign finance reports show he has raised $713,262 for an unspecified statewide office.
Among his major contributions is $125,000 in January from James Stowers -- founder of the Stowers Institute for Medical Research in Kansas City. Stowers financed most of the $30 million campaign for last year's narrowly approved constitutional amendment protecting the ability to conduct stem-cell research.
Associated Press Writer Sam Hananel in Washington, D.C., contributed to this report.