* Romney is Obama's 'biggest nightmare,' says Ros-Lehtinen
* Romney notes his own father was born in Mexico (Adds detail, quotes, byline)
By Jane Sutton
MIAMI, Nov 29 (Reuters) - Three influential Cuban-American Republicans endorsed Mitt Romney as the party's presidential nominee on Tuesday in a move aimed at boosting his support among the party's conservative and Hispanic voters.
"The three amigos are back!" Representative Ileana Ros-Lehtinen said as she joined Representative Mario Diaz-Balart and his brother, former Representative Lincoln Diaz-Balart, to make the endorsement at a food company warehouse in Miami.
Ros-Lehtinen told reporters that she liked all the party's presidential candidates but threw her support to Romney because "he's the most electable."
Flanked by crates of guava paste and coconut water, Romney accepted the endorsement at the warehouse of Conchita Foods, a family-run food company founded by Cuban exiles.
The three conservative Republicans are prominent figures in Miami's 850,000-strong Cuban-American community, voters who could play an important part in the primary election on Jan. 31 in Florida, the fourth-largest U.S. state.
Romney has steadily polled in the top tier of Republican candidates as several of his competitors have surged ahead then fallen back in the race for the party's nomination to challenge Democratic President Barack Obama in the November 2012 election.
But the former venture capitalist has failed to persuade conservative Republicans to get on board. Many are skeptical of his tenure as governor of liberal-leaning Massachusetts, where he helped develop a state healthcare plan that inspired Obama's federal one, which is widely reviled among Republicans.
Some evangelical Christians, an important part of the Republican base, view Romney's Mormon faith with suspicion.
Romney's campaign was dealt a blow on Sunday when New Hampshire's largest newspaper, the New Hampshire Union Leader, endorsed rival Newt Gingrich in the early-voting state where Romney is expected to do well.
But polls have also shown Romney with a better chance of beating Obama in the general election than his Republican counterparts.
"I think Mitt Romney is Barack Obama's biggest nightmare and it's shown by the attack ads," Ros-Lehtinen told reporters.
The Democratic National Committee released television and video ads on Monday making light of Romney's changing positions on issues such as abortion, gay rights, healthcare and the 2009 economic stimulus plan.
They depict Romney as "Two men trapped in one body" - the old Romney who appealed to progressives as Massachusetts governor and the current one courting Tea Party conservatives as a presidential candidate.
At the Conchita Foods warehouse on Tuesday, Romney signaled he was ready to go head-to-head against Obama.
"Bring it on. We're ready for them," he said.
The food company's owners, the Ferro family, fled to the United States in the early 1960s after Fidel Castro's communist government seized the food cannery they had built in Cuba during the 1930s and modernized with U.S. government financing during World War Two.
Romney compared his own background to theirs.
"My dad was not born in this country. He was born in Mexico," Romney said, adding that his grandfather moved the family to the United States when his father was a boy because "he believed this was the land of opportunity."
Romney said his father, George Romney, started out as a poor carpenter who sold paint out of the trunk of his car but rose to become chairman of an auto company and governor of Michigan through hard work.
Obama is thwarting other Americans' efforts to advance economically, he said.
"I don't think he understands America ... or how its economy should work," Romney said. "He never had a job. I think to create jobs it helps to have had a job." (Editing by Mohammad Zargham)