Missouri Secretary of State Robin Carnahan’s (D) announcement Tuesday that she will run for Senate in 2010 sets the stage for what is expected to be one of the blockbuster races of this election cycle.

Carnahan is seeking the seat held by retiring Sen. Kit Bond (R-Mo.), who said last month that he would not run for a fifth term.

While Carnahan’s announcement essentially cements the Democratic field, the Republican side of the contest remains more fluid.

Rep. Roy Blunt (R) is seriously mulling the race, and a source close to the former House Minority Whip said he is still in the decision-making process and doesn’t appear to be in a hurry to make an announcement.

“He’s making calls, he’s talking to people, people are talking to him,” the source said. “He’s seeing what the best course of action is right now for him and the state.”

Carnahan made her announcement — which has been widely expected — in an online video.

“Our country is facing tough economic times and threats to our security like never before,” Carnahan said, looking straight into the camera. “It’s time we had elected leaders ready to stop the political bickering and start solving problems.”

National Democrats quickly coalesced behind Carnahan, the daughter of late Gov. Mel Carnahan (D) and former Sen. Jean Carnahan (D), and the sister of Rep. Russ Carnahan (D).

EMILY’s List, who has supported Carnahan in her past two statewide bids, moved quickly to endorse her Tuesday.

“As secretary of state, Robin has stood out, working to reform elections in Missouri and tackling voter fraud,” said Ellen Malcolm, president of EMILY’s List. “Her achievements were recognized in 2008 when she earned more votes than any other statewide Democratic candidate in Missouri history.”

Democratic Senatorial Campaign Committee Chairman Bob Menendez (N.J.) called Carnahan the best candidate that they could have hoped for to run for the Senate seat.

Missouri has been a bellwether state when it comes to national elections, and although it remains conservative in nature, Democrats have made gains there in the past two election cycles.

In 2006, now-Sen. Claire McCaskill (D) defeated then-Sen. Jim Talent (R), and in 2008, popular state Attorney General Jay Nixon (D) cruised to victory in the open-seat gubernatorial race. However, in last year’s presidential contest, Sen. John McCain (R-Ariz.) eked out a narrow victory over now-President Barack Obama.

Senate candidates are expected to raise in the neighborhood of $20 million each to wage a competitive campaign next year.

Blunt reported having $336,400 in his campaign account at the end of 2008, which gives him a slight fundraising head start over Carnahan or any potential GOP primary opponents.

Jared Craighead, executive director of the Missouri Republican Party, said that even if Blunt passes on a Senate bid, the party has a deep bench of other candidates. However, he added, it seems that Blunt is serious about running.

“I think everything that I’ve seen Roy doing in terms of his activity leads me to believe that he is very, very seriously considering bid for the United States Senate,” Craighead said.

Blunt is not the only Republican looking at the race. Both he and former state Treasurer Sarah Steelman have met with the National Republican Senatorial Committee about running. National Republicans, however, believe Blunt would be a stronger candidate.

Talent is also looking at running, but according to Missouri GOP sources, he would not challenge Blunt in a primary.