Iowa State Sen. Joni Ernst, who is running for the U.S. Senate, has been endorsed by Mitt Romney and Sarah Palin. (Photo by Dan Balz/The Washington Post)

Pop quiz: What do Mitt Romney and Sarah Palin have in common? Hint: It's the same thing the U.S. Chamber of Commerce and Senate Conservatives Action have in common with Sen. Marco Rubio (R-Fla.).

The answer is they all back state Sen. Joni Ernst (R) in Iowa's U.S. Senate race. Tea party and establishment Republicans with a history of butting heads have coalesced behind Ernst's candidacy with less than a week to go until the June 3 primary. In an election cycle when tensions have been on full display in numerous races, Iowa is one of a handful of places where there has been surprising synergy between the two wings of the GOP.

Both Senate Conservatives Action and the Chamber of Commerce are on the air for Ernst this week as the frontrunner tries to fend off a well-heeled ad blitz from wealthy self-funder Mark Jacobs (R). The two groups were on competing sides of the recent GOP primary in Idaho's 2nd district. There, Chamber-backed Rep. Mike Simpson defeated attorney Bryan Smith, who was backed by the Senate Conservatives Fund, which is affiliated with SCA.

Romney will campaign for Ernst later this week. Palin stumped for her earlier this year. The two Republicans have very different bases of support.

Meanwhile, Rubio, an example of a Republican who blurs the line between tea party and the establishment, is pitching in for Ernst with an ad from his political action committee.

Iowa isn't the only Senate race featuring such harmony. In Alaska, the chamber and the anti-tax Club for Growth have united around former attorney general Dan Sullivan (R). In Arkansas, establishment forces and conservative organizations got behind Rep. Tom Cotton (R) very early in the 2014 cycle.

Why it happens some places and not others is open to debate. There is incentive for both conservative and moderate groups to get behind capable candidates like Ernst, even though she is neither a prototypical tea party hero nor a natural establishment icon. The fact that she can win simply makes her more appealing to Republicans of different stripes.

Another school of thought is that Republican candidates, informed by missteps others made during the last two election cycles, have learned to do a better job of attracting both conservative and moderate GOP voters and donors.

Of course, there's no guarantee it will work. If Ernst does not clear 35 percent in Tuesday's vote, the nomination process will move to a convention. And while Sullivan is the clear frontrunner in Alaska, he still has primary opponents.

In other words, the question of whether harmony equals more wins for the GOP has yet to be settled. But 2014 should go a longer way toward answering that question than 2012 or 2010 did.