(Steven Senne/AP)

Hillary Clinton will headline retiring Sen. Tom Harkin's annual steak fry in Iowa next month, according to a Democratic source familiar with event details, making her first visit to the Hawkeye State since losing the 2008 presidential caucuses.

Clinton's return to Iowa is sure to stoke speculation about her potential 2016 presidential campaign and will serve as a high-profile kickoff for her return to the campaign trail this fall to help Democrats in the midterm elections.

Clinton and her husband, former president Bill Clinton, will address the Harkin steak fry, which will be held Sept. 14 in Indianola at the Balloon Field. Rep. Bruce Braley, the Democratic nominee to succeed Harkin in the Senate, also will speak at the event.

“I couldn't be happier or more thrilled that both Bill and Hillary are coming out," Harkin said in an interview with The Washington Post. "It is going to be a shot in the arm for Democrats in Iowa."

Harkin added, "I think you will find at the steak fry that Iowans love Hillary — and Bill. I think you're going to be able to feel it. Iowans have a great deal of love and respect for both of the Clintons. Both Bill and Hillary have provided great leadership for our country and to our party in the past and I'm sure they will continue to do so into the future.”

Clinton spokesman Nick Merrill said, "She’s looking forward to campaigning for her Democratic friends and colleagues and to helping the effort to move America forward, including a stop to see her old friend and colleague, Senator Harkin, to help raise money for important races in Iowa."

The event — Harkin's 37th annual and final steak fry — will serve as a capstone for the beloved Iowa populist who is retiring after 30 years in the Senate.

The night before the steak fry, Harkin is planning a reunion for all of his staffers dating to 1975, when he first went to Washington as a House member. More than 100 staffers are planning to attend the reunion, which will take place Saturday night at Principal Park, the downtown Des Moines baseball stadium where the minor league Iowa Cubs play.

For months, Harkin had kept the identity of his headliners under wraps, but Iowa had been abuzz with rumors that Hillary Clinton might appear. Harkin and his wife, Ruth, have been friends with Bill and Hillary Clinton for decades, and Harkin said he first approached the Clintons about appearing together at his final steak fry many, many months ago.

“I said, 'This is my last one, and you two are two people we love, we admire, we respect, and it would sure be a real great honor for me to have this for my last one,' " Harkin said in the interview. “They are excited about it. ... It’s usually Bill or it’s Hillary, but Hillary and Bill, Ruth and Tom, we’ve been friends a long, long time, both personal and professional.”

Jeff Kaufmann, chairman of the Republican Party of Iowa, issued a statement saying: "I welcome Hillary Clinton to Iowa after more than six-and-a-half years flying over the Hawkeye State. In the spirit of bipartisanship, I hope potential presidential candidates from both parties visit Iowa early and often to interact with Iowa voters in a meaningful way."

For Clinton, Iowa has been a difficult proving ground. She campaigned heavily in the state in the run-up to the 2008 caucuses, but finished a disappointing third behind winner Barack Obama and runner-up John Edwards.

Although Clinton has stayed away from the state in the years since, Ready for Hillary, a pro-Clinton super PAC, has been organizing in Iowa since the start of this year.

Scott Brennan, chairman of the Iowa Democratic Party, said in a statement: "We are so excited to welcome President Clinton and Secretary Clinton to celebrate the final Harkin Steak Fry this year.  We are thrilled to be able to honor Senator Harkin’s legacy and all that he has done for Democrats and for Iowans. This is a great time for our party — Democrats from across the country are coming together to make sure that we deliver results for Iowans and elect Bruce Braley, Jack Hatch and the rest of our great Democratic ticket.”

— Dan Balz contributed to this report.