Former Republican presidential nominee Mitt Romney greets supporters before speaking at a rally for Iowa Republican Senate candidate Joni Ernst, left, Friday, May 30, 2014, in Cedar Rapids, Iowa. (AP Photo/Charlie Neibergall)

This much is clear: Iowa state Sen. Joni Ernst is poised to finish first in Tuesday's Republican primary for U.S. Senate.

What isn't clear: Whether she will win more than 35 percent of the vote, thereby claiming the nomination outright. A pair of surveys released during the last two days show Ernst is hovering right around the magic number.

Suffolk University's new survey of three bellwether counties, released Monday, shows Ernst is pulling between 36 and 45 percent of the vote there. A Des Moines Register statewide poll released Sunday shows Ernst leading businessman Mark Jacobs 36 percent to 18 percent.

Both GOP establishment (think Mitt Romney and the Chamber of Commerce) and tea party (think Sarah Palin and Senate Conservatives Action) forces have lined up behind Ernst. Jacobs is a lone wolf using his personal wealth to stay in the mix.

If no candidate receives more than 35 percent of the vote, the nomination will be decided by some 2,000 Republican activists at a June 14 state convention. A convention would be bad news for Senate Republicans for a few reasons.

First, it would put Ernst -- who most of the party's influential figures believe would the strongest general election candidate of the bunch -- at risk of not winning the nomination at all.

Second, more time and resources devoted to an extended primary means more time and resources not trained on Rep. Bruce Braley, the likely Democratic nominee in the race to replace retiring Sen. Tom Harkin (D).

Third, activists would be able to nominate someone who isn't on Tuesday's ballot. Okay, so the odds they would go that route seem pretty remote. But even the off chance of an unpredictable convention producing a surprise nominee is enough to make leading GOP figures sweat after months of support for Ernst's candidacy.