LOS ANGELES — As Republicans troop through New Hampshire, the scene of the first-in-the-nation primary, the latest poll shows just how heavy the lift will be for some of the front-runners.

According to a poll by the Pew Research Center for the People & the Press released today, just one in four registered voters has an excellent or good impression of the aspirants, while sizable majorities said they could not vote for any of the top four candidates vying for the 2012 GOP presidential nomination.

The findings come as Mitt Romney formally declared his desire to be president and Sarah Palin, the Republicans’ 2008 vice presidential nominee, was heading to New Hampshire as part of her bus tour of historical sites in the Northeast.

The Pew poll also shows that President Barack Obama continues to hold a commanding lead over a generic Republican opponent, 48 percent to 37 percent, virtually unchanged from March. At a comparable period, April 2003, President George W. Bush had a 48 percent to 34 percent lead over a generic Democrat. Bush had a 72 percent approval rating at the time while Obama’s is 52 percent.

The GOP’s problem is that there is no generic candidate, nor any candidate who seemingly unites the different conservative streams in the party: economic, social and libertarian. In addition, significant parts of the party have adopted their version of the ABM race: Anybody But Mitt, even though Romney is the front-runner in most polls.

Romney seems to alienate fewer voters than his party’s other choices, according to the Pew poll. About 44 percent said they would never vote for the former Massachusetts governor, compared with other best-known candidates. Sixty-three percent said they would spurn Palin and former House Speaker Newt Gingrich. About 60 percent said there was no chance they would vote for Texas Rep. Ron Paul, who is waving the libertarian flag in the race, and 54 percent said they could not support former New Mexico Gov. Gary Johnson, another libertarian voice.

Businessman Herman Cain continues to show some strength, as he has in other recent surveys, but his position is limited by how few people have heard of him. Only 44 percent of Republican and Republican-leaning voters said they knew of Cain, but 74 percent of that group said there is at least some chance they would vote for him.

Overall, Republican and Republican-leaning voters were mixed on their field, which explains why some Republicans are pushing for more candidates to stand up. About 44 percent said the possible candidates are excellent or good, but 43 percent said the party’s candidates are only fair or poor.

Religion continues to play a significant role that could have an impact on Romney and former Utah Gov. Jon Huntsman. About 25 percent said they would be less likely to support a candidate who is Mormon, about the same as four years ago. Compared with other religious groups, more white evangelical Protestants – a key GOP voting bloc particularly in the early states – said they were less likely to back a Mormon.

The latest survey was conducted May 25 to 30 among 1,509 adults. The margin of error for the full sample is plus or minus 3.5 percentage points, but the sub-groups can vary up to plus or minus 6 percentage points.