BOSTON -- Massachusetts U.S. Rep. John Tierney is the latest in a line of Bay State politicians to say he is not running for the U.S. Senate seat recently left vacant by the death of Sen. Ted Kennedy, while U.S. Rep. Michael Capuano says he's "99.9 percent there."

In a statement released Monday morning, Tierney said although many people have urged him to run for the seat, he feels he can best serve the state by remaining in the House.

"Since the passing of Senator Kennedy, a great number of people have kindly urged me to run for the United States Senate. Such an opportunity comes open perhaps only once in one's lifetime. I have given the matter serious consideration, and while thankful for all of the confidence shown, today I am stating that, at this time, I shall not be a candidate in the special election for the U.S. Senate," Tierney said in a prepared statement.

The 6th District Democrat from Salem cited his seniority on several Congressional committees, such as the Education and Labor Committee, as part of the reason he decided to hang on to his House seat.

"My seniority, position on committees, strong relationships, and experience create the ability to weigh in on significant matters," Tierney said.

Tierney's colleague in the House, U.S. Rep. Ed Markey, has also decided against a run for the Kennedy seat.

Massachusetts Attorney General Martha Coakley has announced her candidacy.

U.S. Reps. Stephen Lynch and Michael Capuano have also pulled nomination papers.

"I'm still talking to a few people who are still left on my list ... to make sure that they think this is a do-able race. As I've said from the beginning, I'm not interested in getting involved in something that is impossible. I will be the underdog and I understand that. Nonetheless, it's an opportunity to really continue the legacy of Ted Kennedy in a progressive manner, in an activist manner," Capuano said.

He said there are deep and serious philosophical differences between himself and Lynch and Coakley.

Capuano said he will make a final decision by the end of the week.

State Sen. Scott Brown has also officially declared his candidacy as a Republican.

The primary in the race is Dec. 8 and the special election is scheduled for Jan. 19.

Kennedy died of brain cancer in Hyannis Port on Aug. 25 at the age of 77. He had held the seat for close to 50 years.