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.