(CNN) - The field is growing among Republicans who want to take on Claire McCaskill - the first-term senator and strong Obama ally from Missouri.
Ed Martin, an attorney and former chief of staff to Gov. Matt Blunt, announced Monday he is adding his name to a race that will be among the most closely-watched Senate contests in 2012.
"I believe the 2012 election will come down to a very simple question," Martin said in a Web video announcing his candidacy. "What should the size, scope and cost of the federal government be. President Barack Obama and Sen. Claire McCaskill have answered this simple question. They've said the federal government should be bigger, it should be more powerful and it should be more costly."
Martin was supported by several Tea Party organizations in his failed bid last year to unseat Democratic Rep. Russ Carnahan.
Missouri Republican Treasurer Sarah Steelman has also announced a Senate bid and Ann Wagner, the former Missouri Republican chairman and candidate to head up the Republican National Committee is also mulling run.
Meanwhile former Sen. Jim Talent, the man McCaskill narrowly defeated in 2006, ruled out a bid of his own last week.