Sen. Evan Bayh (D-Ind.) has never signaled a love for lawmaking while in the Senate, one of his Democratic colleagues said.

Sen. Claire McCaskill (D-Mo.) said that Bayh's retirement announcement on Monday, which came as a surprise to most political observers, hardly shocked her.

"He has been a very independent voice, and he's been very frustrated," McCaskill said, as reported by the St. Joe News. "He really has never sent a signal, since I've been part of the Senate, that he loved being part of the club."

McCaskill, a centrist much like Bayh who was elected in 2006, has served with her Indiana colleague for just over three years in the Senate.

Many pundits were taken aback by Bayh's announcement, given his lead in the polls and considerable fundraising reserve.

But McCaskill said she was unsurprised.

"I have to be candid and say it didn't shock me," she told Missouri reporters during a conference call. "I think Evan had just had enough."

McCaskill's lack of surprise differs from some other Senate Democrats, including Majority Leader Harry Reid (D-Nev.), who reportedly didn't receive a call from Bayh until news of the Indiana senator's retirement had already leaked.