With less than two weeks until Iowa’s congressional primaries, it’s still unclear whether a Republican Senate candidate can clear the 35 percent vote threshold needed to win the nomination outright.  

In an attractive pickup opportunity for national Republicans, there are four major candidates seeking the nomination for the seat of retiring Democratic Sen. Tom Harkin. The leading contenders are state Sen. Joni Ernst, who has the tacit support of the governor and earned national attention with an ad about castrating hogs, and Mark Jacobs, a self-funding former energy executive. Radio host Sam Clovis and former U.S. Attorney Matt Whitaker round out the top four.  

A recent poll found Ernst with 31 percent support — in the lead and within striking distance of the nomination. But Jacobs, who outspent Ernst by more than 5 to 1 through March, has dominated his opponents on the airwaves, and a potential late surge by Clovis could spread the vote around and force the nomination process to a convention, where anything can happen.  

“I think either Ernst or Jacobs will get to 35 percent,” said Craig Robinson, a former political director of the Iowa Republican Party and editor-in-chief of The Iowa Republican politics blog. But, he continued, “Clovis has plenty of room to grow” and can improve his standing if he is more “aggressive in drawing distinctions between himself and the other candidates” and seizing the “social conservative mantle.” Former Iowa Republican Party Chairman Matt Strawn also believes either of the top two contenders will advance on June 3, mostly because of the lack of a threat behind them.  

“I just don’t see Clovis and Whitaker with enough financial firepower to take advantage of Ernst and Jacobs blowing each other to bits over the campaign’s final weeks,” Strawn said.  

Ernst has collected a series of endorsements from a unique combination of people and groups since her now-famous ad went viral in March, raising her profile beyond that of an average state legislator. Both Mitt Romney and Sarah Palin are backing Ernst, as are the U.S. Chamber of Commerce and Senate Conservatives Fund.  

But with that attention has come extra scrutiny of her record. When Ernst recently declared to the Des Moines Register she still had reason to believe there were weapons of mass destruction in Iraq, she was forced to walk that back after a flurry of negative press. Ernst also disavowed her own vote to raise the gas tax — not something to tout in a Republican primary — at a candidate’s forum in Bettendorf, Iowa, on Monday night.  

“I made a mistake,” Ernst said, when the subject came up. “I made a mistake on that one.”  

Still, Ernst appears to be the front-runner headed into the primary. Democrats seem to see it that way as well, as the campaign of Rep. Bruce Braley, who has no primary opposition, released a memo Wednesday tying Ernst to the “radical, right wing” positions of conservative Rep. Steve King, R-Iowa, who had considered a Senate bid.  

“At the end of the day, with Ernst, Republicans get none of the advantages of a Steve King candidacy and all of the downsides,” Braley senior adviser Jeff Link wrote. “It’s hard to imagine how the long, drawn out Republican primary contest could turn out any worse for the Iowa GOP.”  

While Ernst and Jacobs scrape to clear 35 percent, Iowa Republicans said Clovis is the candidate to watch. Trudy Caviness, chairwoman of the Wapello County Republican party central committee, said that until recently she would have agreed that it was solely a race between Jacobs and Ernst.  

“But in the last two or three weeks, Clovis has really become strong in the grass roots” and with the tea party, she said, which could help him compensate for paltry fundraising.  

Last week, Clovis got two big endorsements: one from Bob Vander Plaats, the influential president of the socially conservative Family Leader organization, and one from former Sen. Rick Santorum, R-Pa., who won the 2012 Iowa caucuses.  

“That’s essentially telling every social conservative in Iowa to vote for Clovis,” said one Iowa Republican, who is publicly neutral in the race.  

If no candidate gets more than 35 percent in the primary, the race goes to a June 14 state convention, where some 2,000 delegates — selected through a series of caucuses and local conventions earlier this year — vote to decide the party’s nominee. The delegates are more politically involved than an average off-year primary voter.  

They are “the most dedicated grass-roots activists across the state in all 99 counties,” said Jamie Johnson, a member of the GOP’s state central committee.  

This year they may also be a Gov. Terry E. Branstad-friendly crowd, sources said, which could give Ernst an edge. While Branstad has remained publicly neutral in the race, Lt. Gov. Kim Reynolds has endorsed Ernst.  

Still, conventions are unpredictable affairs. Chuck Laudner, Clovis’ campaign manager, has some experience winning conventions as part of the team that helped King win a convention in his first bid for Congress in 2002.  

Beyond that, this select group of people is “far more likely to choose a candidate based on conservative values rather than their placement in the results of the June primary,” Johnson said. And that could give an edge to Clovis, who has positioned himself as the socially conservative candidate in the race.  

The winner will face Braley in a race still outside the top tier of the competitive Senate playing field. Braley’s critical remark about Republican Sen. Charles E. Grassley at a fundraiser earlier this year notwithstanding, Democrats remain favored at this point. The outcome of the GOP primary will undoubtedly affect the strength of that advantage.  

The race is rated Leans Democrat by the Rothenberg Political Report/Roll Call.