DES MOINES — U.S. Sen. Tom Harkin endorsed Rep. Bruce Braley to be his successor on Saturday.

The longtime Democratic senator made the announcement in Des Moines.

Harkin called him a "passionate progressive" and a "natural-born leader."

The endorsement is probably not a surprise.

Braley, a Waterloo Democrat, has been raising money and building support for the 2014 campaign, snaring the endorsements of elected Democrats and unions across Iowa. He's the only Democrat to have said he's running, although the filing deadline for primaries isn't until next year.

Harkin announced in January that he would not seek another term. Shortly after, Braley said he would run for the office.

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A handful of Republicans are considering a bid, including Rep. Steve King, R-Iowa, Lt. Gov. Kim Reynolds and Secretary of Agriculture Bill Northey.

Braley said it was an honor to be endorsed by "my friend and mentor."