Cullen Sheehan, Coleman's former campaign manager, said Republicans are eager to take Franken on in 2014.
"In a state like Minnesota, a guy like Franken who's as liberal as he is and who's as partisan as he is, is always going to be vulnerable," Sheehan said.
Franken took the oath of office last July 7, a week after the Minnesota Supreme Court upheld a ruling that Franken got the most legally cast votes and Coleman conceded.
Franken said he has seen Coleman only once since then. They bumped into each other at a Martin Luther King Jr. Day breakfast in Minneapolis in January. Coleman confirmed the encounter.
"He said, 'It's a great job, isn't it?'" Franken said. "And I go, 'Yeah, it really is.'"