Pfeffer announced on August 23, 2005, that he would be entering the primary. A former Democrat, he supported George W. Bush in 2004 and became a Republican in 2005. In his campaign announcement, Pfeffer focused mainly on border controls with Mexico. He criticised Bingaman in comparison to his own support for reform of the Social Security system and the Iraq War as well as U.S. relations with China, saying "With all due respect, I do not believe the present occupier of the junior seat from New Mexico is doing all that can and should be done on these fronts," he said of Bingaman. "I believe I can do a better job..." Pfeffer also commented that he would have a hard time raising an amount equivalent to Senator Bingaman, a problem faced by any of the latter's potential challengers.
Bingaman had a 60% approval rating in one poll. He faced no primary opposition. There had been speculation that Bingaman would give up the chance to run for another term to pursue a lobbyists' job in Washington.