US conservatives are invoking apocalyptic imagery to trash President Barack Obama's "terrible" record as they tamp down concern over which Republican might face the incumbent in November.

The party's star power is on full display at this year's Conservative Political Action Conference (CPAC) held over three days in Washington, with the top Republican presidential candidates set to speak on Friday.

But as frontrunner Mitt Romney and rivals Newt Gingrich and Rick Santorum battle it out in an increasingly bitter race, the event's featured lawmakers and luminaries sought to project unity in their desire to unseat Obama.

Senator Marco Rubio, who has been widely discussed as a possible Republican presidential running mate, charged Obama with putting the US economy into a deeper hole, tying the hands of big business, infringing on citizens' rights and instituting unpopular health care reforms.

"The president of the United States looks like he is a really good father, a really good husband. But he is a terrible president," Rubio said to a standing ovation.

Senate Republican leader Mitch McConnell also took aim at Obama's economic record, accusing him of being "completely and totally AWOL" when it comes to reining in the country's massive debt.

"We're not in this mess because of a tsunami in Japan or a debt crisis in Europe," McConnell said. "We're in it because (Obama) got everything he wanted for two long years.

"When November comes around, voters will know who was in charge -- they'll know this president's record, and after that he'll have as much time to play golf as he wants," he said, referring to this year's presidential election.

In one animated speech after another, conservatives highlighted Obama's shortcomings, using increasingly apocalyptic metaphors.

"We are on the precipice of an abyss," said Colin Hanna, who heads the grassroots group Let Freedom Ring.

Senator Mike Lee invoked conservative icon and former president Ronald Reagan's image of America as "a shining city on the hill," and warned that after four more years of Obama it could "become a decaying city in a hole."

Republican congresswoman Michele Bachmann, a presidential candidate for months until she dropped out in January, savaged Obama's foreign policy and said he "made a mess" of the Middle East.

Senator Rand Paul, whose father, Texas congressman Ron Paul, is the fourth Republican vying for the White House, suggested Obama hates rich people, saying "the president continues to roam the country blaming millionaires and billionaires" for the economic woes of the American middle class.

Jeffrey Lord, a former Reagan advisor, told CPAC that Obama was "the greatest gift to conservatives" because he would unite them against him.

That has yet to happen. While Romney leads in the all-important delegates count, he slipped up badly this week, losing a trio of state contests to Santorum, and there is mounting skepticism about his conservative bona fides.

David Keene, president of the powerful National Rifle Association and a board member of the American Conservative Union, which hosts CPAC, said the party is tentative about Romney.

"In the primaries he's been drawing more and more conservative support, but he still hasn't sealed the deal with a lot of the activists," he told AFP.

"But the one thing that all of these people have to unite them, whichever one of these guys is nominated, is (the goal of defeating) Barack Obama," he said.

Experts and even top US lawmakers have warned that the Republican race could last all the way to the party's national convention in late August, which would be a massive drain on resources that could be better spent targeting Obama.

Rubio in his speech sought to tamp down concern that conservatives had yet to coalesce behind one man.

"We have four really good candidates," he said. "At the end we're all going to come together, and we know that right?"