An uphill climb for Senate Republicans in Iowa may be further complicated by a convention process that local Republicans fear will nominate an unpalatable candidate for a statewide race.

Democratic Sen. Tom Harkin’s retirement created a prime pickup opportunity for Republicans in Iowa, a perennial swing state. But after several top-tier Republicans declined to run, a cast of lesser-known GOP candidates announced their interest.

As the four-candidate Senate field grew last month, so did the likelihood that Hawkeye State Republicans would have a rare nominating convention for Congress. If no Republican gets 35 percent of the vote in the primary, a convention picks the nominee — a process so unpredictable it gives GOP operatives heartburn.

“This race is winnable, but if the wrong person is chosen at convention, it would make it impossible,” one top Iowa Republican operative said.

It would be the second time in 50 years that Iowans used a convention to pick their nominee for Congress. The last time was 2002, when Rep. Steve King won the nomination for a conservative House seat in northwest Iowa.

The stakes would be much higher for a competitive Senate race against the likely Democratic nominee, Rep. Bruce Braley. Four Republicans have announced they’re running so far: David Young, former chief of staff to Republican Sen. Charles E. Grassley, former U.S. Attorney Matt Whitaker, conservative radio show host Sam Clovis and attorney Paul Lundby.

Republicans say two others — state Sen. Joni Ernst and former oil executive Mark Jacobs — will make their official entrances in the coming weeks.

Ernst is an ally of Iowa Gov. Terry E. Branstad, while Jacobs earns plaudits for his marketable life story, and Young has a strong statewide network. Longtime Iowa GOP operatives see that trio as the party’s most promising picks for a general-election audience.

But those same operatives fear the growing faction of libertarians in the state party could skew the convention and elect a wild-card nominee. Branstad’s team especially wants to avoid that scenario because he will share the November ticket with the Senate nominee.

A six-candidate field would almost ensure no candidate gets enough support in the June 3 primary to avoid a nominating convention. Instead, the nomination would be decided by 2,000 delegates with the time, resources and desire to see the lengthy convention process through to the end.

The convention process begins in January with precinct caucuses, which then go to a March county convention, a districtwide convention in April and ultimately a statewide convention on June 14, 2014.

But party officials have incredible leeway to set up and schedule the convention process. They can determine participant registration, convention date and location, plus how delegates pick the nominee.

Last cycle, supporters of former Rep. Ron Paul, R-Texas, took over much of the state GOP’s leadership that makes these decisions. The current chairman, co-chairman and finance chairman of the state party are supporters.

As a result, there’s some concern the rules could be changed to favor a libertarian candidate in the convention process. Operatives caution that, because of this, Republicans understand they must turn out supporters in the low-turnout delegate selection process in a midterm cycle.

In 2010, about 1 out of every 3 people who attended a precinct caucus had the opportunity to be a delegate at the state convention, according to Matt Strawn, the chairman of the Iowa Republican Party at the time. The result is a massive organizational effort by candidates up and down the ballot.

“The off-year caucus provides a great opportunity for Senate campaigns to make sure their supporters are filing delegate slots because participation is historically low,” Strawn added.

While the Republicans battle through the summer, Braley can continue to raise funds for the fall of 2014. He’s running in an uncontested Democratic primary and is widely considered the front-runner with $2 million in the bank.

Democrats argue that Republican fears of an unelectable convention nominee signal the GOP’s attempt to blame the process, instead of faulty candidate recruitment, for a potential loss.

“This is an early attempt to place blame at the feet of the convention attendees a year from now, rather than the fact that they couldn’t get anybody good to run,” said Travis Lowe, a Democratic consultant and Iowa native. “They’re setting the table very early for, ‘It’s not our fault.’”

To be sure, some of these candidates may drop out before the filing deadline in March. Fewer candidates means a great likelihood that Republicans avoid a convention.

At least that’s what some GOP operatives hope will happen to help the party’s chances of picking up this seat.

“Democrats would like to assume that a convention is a disaster for us,” said David Kochel, an Iowa Republican consultant. “It could be, but it also may not be.”