(Reuters) - Nearly 60 percent of Americans would never support a Republican presidential bid by former Alaska Governor Sarah Palin or real estate mogul and reality TV star Donald Trump, according to a new poll on Wednesday.

Trump, who has been testing the waters for a possible 2012 run for the Republican presidential nomination, has said he would announce something before June -- after his reality TV show, "Celebrity Apprentice," ends its season on May 22.

Palin was the 2008 Republican vice presidential nominee and is keeping her supporters guessing on whether she will run.

The Quinnipiac University poll of 1,408 voters found that about half would consider or be enthusiastic about backing former Massachusetts Governor Mitt Romney or former Arkansas Governor Mike Huckabee in the November 2012 election.

"Mitt Romney and Mike Huckabee are in the best shape. Sarah Palin and Donald Trump suffer from the reality that, as our mothers told us, 'You never get a second chance to make a first impression,'" said Peter Brown, assistant director of the Quinnipiac University Polling Institute.

Among the 613 Republican and independent Republican-leaning voters, the poll showed Romney as favorite to win the Republican presidential nomination with 18 percent, followed by Huckabee and Palin with 15 percent and Trump with 12 percent.

Indiana Governor Mitch Daniels and former House of Representatives Speaker Newt Gingrich each have 5 percent, while former Minnesota Governor Tim Pawlenty and Minnesota Representative Michele Bachmann and both on 4 percent.

The margin of error for that subset was plus or minus 4 percentage points.

The telephone poll, conducted between April 16 and May 1, was released on the eve of a debate in South Carolina among a handful of potential Republican candidates, none of them high-profile names.

The error margin for the larger group was 2.6 points.

(Reporting by Michelle Nichols; Editing by Doina Chiacu)