Rep. Bruce Braley (D-Iowa) announced Thursday that he will run for the open Senate seat left by retiring Sen. Tom Harkin (D-Iowa).

Braley first noted his plans in a post on his Facebook page showing his campaign logo. He later said in an e-mail that he is forming a campaign committee.

"It's a big responsibility to represent the people of Iowa in the United States Senate, especially after Tom Harkin has shown us how for the last 30 years," Harkin said in the e-mail. "But, if you are willing to help me, I'm ready to go."

Braley posted this campaign logo on his Facebook page.

Braley's entry into the race has been expected since Harkin announced his retirement last month. The congressman has been thought to be Democrats' next big statewide candidate for some time now.

There has been some talk that Agriculture Secretary Tom Vilsack or his wife, former congressional candidate Christie Vilsack, could also run. But thus far the Vilsacks have shown little interest in the race.

competitive primary on the GOP side seems more likely, with conservative Rep. Steve King looking like a sure bet to run and Rep. Tom Latham, who is allied with House Speaker John Boehner (R-Ohio), also looking at the race.

Updated at 11:19 a.m.