The move also was the first in what could be a reshuffling of Iowa's U.S. House delegation in 2014.
Braley, a four-term House member from Waterloo, announced his plans in an email to potential supporters, describing his goal of representing all Iowa residents as "a big responsibility." The decision came less than two weeks after Harkin, a fellow Democrat first elected to the Senate in 1984, said he had decided against seeking a sixth term in next year's election.
Iowa's first open Senate seat in 40 years is seen as up for grabs by both parties and a chance for Republicans who need a net gain of six seats in the U.S. Senate to seize the majority.
Braley has established strong relationships in Washington since going to Congress and met last week with Senate Majority Leader Harry Reid and leaders of the Democratic Senatorial Campaign Committee, who he said encouraged him to run. He has been huddling with family and talking to Iowa Democrats and advisers since Harkin's Jan. 26 announcement.
"There appears to be a lot of support for Braley," Iowa Democratic Party Treasurer Ken Sagar said. Other possible candidates, including former Iowa first lady Christie Vilsack, have not ruled out running.
King, a six-term House member from Kiron, stoked speculation Thursday by soliciting contributions to his House campaign committee, but he said that was a response to reports that a Washington-based political action committee was planning to air advertisements against him. The group has said it is concerned that the conservative King might have problems with re-election after some Republicans who were expected to win races last year did not. King has made incendiary comments about immigrants and President Barack Obama, and some believe a more moderate Republican would have more appeal.
"It's about whether a person with financial resources can wake up and say someone cannot be a nominee by a political party," King said. "I'm closer to running now than I was yesterday, and each day it gets closer."
Given Iowa's balanced partisan makeup and perennial swing-state status in the past four presidential elections, the Senate race is expected to be hard-fought and expensive. Iowa and national campaign strategists said they expect both candidates to have to raise more than $10 million each, but that television advertising by super PACs could push the total cost of the race to more than $40 million.
Democrats hold a 10-seat edge in the Senate, but they have more seats to defend in 2014 — 20 compared with only 13 for the Republicans. In GOP-leaning West Virginia, five-term Democratic Sen. Jay Rockefeller recently announced he would not seek re-election. It's not certain whether Democrats Tim Johnson will seek a fourth term in Republican-leaning South Dakota or Carl Levin will seek a 7th term in Michigan, where Republicans have done well in recent off-year elections.
Braley, who has a reputation as a populist and consumer watchdog, has proved a robust fundraiser in previous elections. He's also used to opponents with deep pockets. In 2010, he was the target of attack ads costing more than $1.5 million that were funded by American Future Fund, a super PAC led by the GOP fundraiser Ryan.
Braley's departure from the House cleared the way for what could be a tussle among Democratic lawmakers from northeast Iowa. State Sen. Liz Mathis, of Cedar Rapids, state Rep. Pat Murphy, of Dubuque, state Rep. Tyler Olson, of Cedar Rapids, and Iowa Senate President Pam Yochum, of Dubuque, have all said they may run.
The district covers Iowa's northeast quadrant, including the Democratic stronghold of Dubuque and swing counties of Linn and Scott. It also covers a vast, rolling farm country from the Mississippi River west to Marshall County.