Rep. Bruce BraleyBruce BraleyTen years later, House Dems reunite and look forward Trump: Ernst wanted 'more seasoning' before entertaining VP offer Criminal sentencing bill tests McConnell-Grassley relationship MORE (D-Iowa) will run for the Senate seat held by retiring Sen. Tom HarkinTom HarkinDistance education: Tumultuous today and yesterday Grassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream MORE (D-Iowa), he announced in an email to supporters Thursday morning.

"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. But, if you are willing to help me, I'm ready to go," he writes in the email.

Braley has been expected to run for the seat. He recently met with Democratic Senatorial Campaign Committee Chairman Michael BennetMichael BennetOvernight Finance: House votes to repeal arbitration rule | Yellen, Cohn on Trump's list for Fed chief | House passes Russia sanctions deal | GOP centrists push back on border wall funding Tax credits bring much needed relief Senate Dem: No clarity, 'little competence' behind travel ban MORE (D-Colo.) and Senate Majority Leader Harry ReidHarry ReidConservative Senate candidate calls on GOP to end filibuster Ex-Reid aide: McConnell's 'original sin' was casting ObamaCare as 'partisan, socialist takeover' GOP faces growing demographic nightmare in West MORE (D-Nev.) to discuss the race, and had been rumored to be interested in a statewide run for years.

Braley promised to "get things done for Iowa's middle class" in the email.

"It's time to start a conversation with Iowans – about rebuilding the middle class, creating economic opportunity for everyone, and keeping America strong," he said.

Braley, a four-term congressman from the eastern part of the state, has proven to be a tough campaigner. He narrowly held on to his slightly Democratic-leaning district during the 2010 GOP wave despite being a top target, and won by a wide margin in 2012 against a strong challenger.

No Republicans have announced bids for the seat yet, though both Reps. Steve King (R-Iowa) and Tom Latham (R-Iowa) are considering running. Polls show that Braley would start off with a big lead over King, a firebrand conservative, while Latham would make it a close race in the swing state.