By THOMAS BEAUMONT
DES MOINES, Iowa (AP) - 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.
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.
The National Republican Senate Committee has appointed new Texas Sen. Ted Cruz to help identify potential candidates who could win a general election and weed out those who probably couldn't.
The Iowa race will be part of the battle for control of the Senate in 2014. Democrats have a 10-seat margin, but also the burden of defending more seats. Harkin's involves the most dramatic switch from a Democratic safe seat to one that's up for grabs.
In most states, a retirement announcement by a 73-year-old senator would seem less surprising. But Iowa has a tradition of constancy.
Harkin, elected to the Senate in 1984 after 10 years in the House, is Iowa's junior senator, outranked by Grassley, who was elected in 1980. The 2014 Senate election will be the first here without an incumbent since Richard Nixon was president.
Because of the surprise, the field of potential candidates may remain unclear for a while.
Among Democrats, four-term U.S. Rep. Bruce Braley quickly began calling donors and national party activists, and met Wednesday with Democratic lawmakers at the Capitol in Des Moines.
"What I'm focusing on is reaching out to people who can give me unbiased advice about why I should or shouldn't do this," Braley said in an Associated Press interview Wednesday.
Braley has built a reputation as a consumer advocate and has been a loyal House supporter of Obama's spending programs, in the mold of the liberal populist Harkin.
He said he planned to meet in Washington Thursday with Senate Democratic leadership at the invitation of Majority Leader Harry Reid and Sen. Michael Bennet, chairman of the Senate Democrats' campaign committee.
"Bruce has done a really good job making it seem like he would be an heir to Sen. Harkin," said former Iowa Democratic Party Chairman Scott Brennan.
Another possible Democratic candidate, Christie Vilsack, wife of former Iowa Gov. Tom Vilsack and the loser of a U.S. House race last year, is "leaving all options on the table," said Vilsack's congressional campaign manager Jessica Vanden Berg.
After last election's ugly losses by tea party-friendly candidates in Missouri and Indiana, some top Republicans are trying to avoid a candidate identified with the far right.
Some GOP leaders have already approached U.S. Rep. Steve King, an outspoken social conservative, to gently discourage him from running.