GOP presidential nominee Mitt Romney on Wednesday endorsed state Sen. Joni Ernst in the crowded Republican primary in Iowa’s open Senate seat.  

“It is going to take an incredibly strong Republican to defeat the Democrats as they look to defend this open seat that has been in their control for over three decades. Thankfully, Iowa Republicans have such a candidate in Joni Ernst,” Romney said in a mailer endorsing Ernst, according to the Des Moines Register . “As a mother, soldier and proven conservative, Joni has the kind of experience, skill set and discipline it will take to not only win this important seat, but to also take Iowa values to help turn around the mess in Washington.”  

Romney’s endorsement could help Ernst stand out in the crowded, six-candidate GOP field seeking the open Senate seat in the Hawkeye State. The seat is open this cycle because Sen. Tom Harkin, D-Iowa, is retiring.  

The crowded GOP primary could proceed to an unpredictable nominating convention if no candidate garners more than 35 percent in the primary. It’s a scenario Republicans in the state fear could result in a weaker general-election candidate .  

Some of the other Republicans in the field include businessman Mark Jacobs, who is putting his own money into his race, former U.S. Attorney Matt Whitaker and radio show host Sam Clovis.  

Candidates have until March 14 to file the necessary paperwork for a bid. The primary is June 3.  

The winner of the GOP nod will face the likely Democratic nominee, Rep. Bruce Braley, who is the only Democrat in the field.  

Iowa’s Senate race is rated a Lean Democrat contest by Rothenberg Political Report/Roll Call.