DES MOINES - The announcement that veteran U.S. Sen. Tom Harkin would not run for re-election came as such a shock that neither his own party nor the Republicans had given any thought to who might succeed him.
Harkin and his Republican counterpart, Charles Grassley, have been an Iowa institution in the Senate since the 1980s, and Harkin's robust $2.7 million re-election fund suggested that no change was likely soon.
Now, his unexpected retirement has set the stage for a test of how the major political parties will respond to change not only here, but also nationwide, as demographic forces reshape the electorate.
This combination of undated file photos shows Congressmen, from left: Republican Rep. Tom Latham; Democratic Rep. Bruce Braley and Republican Rep. Steve King. The surprise announcement that veteran U.S. Sen. Tom Harkin would not seek re-election left the field of potential candidates to replace him unclear. Braley has announced he is considering a bid while GOP leaders have approached King and Latham about running for the vacant Senate seat.
With a real chance to win, Republicans will have an opportunity to show whether they can reach out to the growing Hispanic population in a closely contested Midwestern state - a major priority since the party lost the 2012 national race with white-only support. In Iowa, Hispanics have almost doubled over the past decade to more than 5 percent of the electorate, and have backed Democrats like Harkin and Barack Obama by a 70 percent margin.
The party will also show whether it can unify its restive factions, especially evangelical conservatives and the business-first establishment, which are sometimes pulling in different directions.
Meanwhile, Democrats will face the challenge of electing someone not named Harkin. The only other Iowa Democrat elected statewide in recent years, Gov. Chet Culver, was defeated soundly by Republican Terry Branstad in 2010.
"The bottom line is 72 hours ago this was a seat we weren't going to win," said Rob Jesmer, who was director of the National Republican Senatorial Committee last year. "And now it's a seat we have a really good shot at winning. "
National conservative and liberal interest groups are expected to invest more than $30 million in the race, far more than has ever been spent here for a nonpresidential campaign.
Iowa's status as a resolutely bipartisan state - split evenly between the parties in the Senate and Legislature - will make it a special target.
"All the ingredients are here for an extraordinarily expensive barnburner with record spending," said Patrick Dillon, former deputy White House political director under Obama.