Former congressman Charlie Bass (R-N.H.) announced Monday that he won't challenge Sen. Jeanne Shaheen (D-N.H.) in 2014.

"Though I am confident that a strong Republican nominee will defeat Democrat Jeanne Shaheen next year, my current personal and professional responsibilities preclude my candidacy," Bass said in a statement. "I sincerely thank all who have called, emailed or encouraged me in person to run."

Previously, former congressmen Jeb Bradley and Frank Guinta also declined to run, as did the man Shaheen beat in 2008, former senator John Sununu.

Former state Sen. Jim Rubens is Shaheen's highest-profile GOP challenger so far. But she is not considered among the most vulnerable senators facing reelection in 2014.

Other rumored GOP candidates include former Massachusetts senator Scott Brown (Mass.), who was defeated in the 2012 election.