Mitt Romney returned to Iowa Friday for the first time since he lost the presidential election in 2012 in an effort to showcase U.S. Senate candidate Joni Ernst to mainstream Republicans, and said he's opening his campaign donor list to the little-known farmer and national guard officer.

"By indicating my support for Joni as well as some other candidates from around the country we're encouraging donors to help these candidates," Romney told The Associated Press after headlining a rally for Ernst four days before the Iowa primary election. "Part of endorsing someone is letting the people who know, and who like me, to get involved in the campaign."

It was Romney's first time back in the state since losing Iowa's six electoral votes to Democratic President Barack Obama in 2012, despite focusing money, ads and travel on the Midwestern swing state. He has made only one other appearance at a campaign stop this year, for Republican candidates in Idaho.

Ernst, from rural southwest Iowa, has built a lead in the five-way race for the GOP nomination to take on Democratic U.S. Rep. Bruce Braley. They are seeking the seat held by five-term Democratic Sen. Tom Harkin, who is not seeking a sixth term. Ernst has emerged atop the large field with catchy ads, and a varied background.

She is the first candidate to receive the support of both the Senate Conservatives Fund, a group that spent millions opposing Senate Minority Leader Mitch McConnell in his Kentucky Senate primary this year, as well as the mainstream Republican-leaning U.S. Chamber of Commerce. Ernst has also been endorsed by tea party favorite Alaska Gov. Sarah Palin as well as more mainstream party leaders such as Iowa Lt. Gov. Kim Reynolds. Iowa Gov. Terry Branstad has quietly backed her.

"She is uniting coalitions from all ends of the spectrum," Reynolds said at the morning rally in Cedar Rapids. Romney was also campaigning with Ernst in Davenport.

The Iowa open seat is one of several important to determining whether Republicans gain the six seats needed to take the majority in the U.S. Senate, which would allow them to more easily thwart Obama's agenda in his final two White House years.

Romney did well in Cedar Rapids and surrounding eastern Iowa in his two presidential campaigns, beginning with second-place finishes in the 2008 and 2012 caucuses. But Friday's events appeared to be looking ahead to the fall campaign, should Ernst win the primary or be nominated at a state GOP convention. If a candidate does not receive at least 35 percent of the vote Tuesday, the Republican nomination would be decided at convention.

Mark Jacobs, a retired energy company CEO from West Des Moines, has worked in the past two weeks to slow the steady rise of Ernst by criticizing her missed votes in the Iowa Senate this year.

Also running are Sioux City college professor and radio host Sam Clovis, Ames businessman Scott Schaben and former U.S. Attorney Matt Whitaker.

While Ernst is leading opinion polls, her national donor base is not as strong as Jacobs, who has spent more than $3 million of his own money on the primary, and the Democrat Braley, who has developed a national fundraising network during his four terms in the House.

Romney took a jab at Braley during his talk to about 300 supporters and more than a half-dozen national media in Veterans Hall in Cedar Rapids. "There's nothing wrong with lawyers. It's just we have enough of them in Washington."

Braley's campaign shot back: "Joni Ernst has benefited from millions of dollars in spending from the oil billionaire Koch brothers and other out-of-state groups," campaign spokesman Jeff Giertz said.

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Romney has also endorsed candidates in Virginia, New Jersey, Idaho, Massachusetts, Florida, Nevada, Louisiana, Kentucky, Montana, New Hampshire, Wisconsin, California, New York, Georgia, Utah, Arizona, Utah, Oregon, North Carolina, Pennsylvania, Colorado and Alabama. He has appeared at several private fundraisers for them apart from the Idaho and Iowa stops for public campaigning.

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