US PRESIDENT Barack Obama accused his Republican rivals of embracing an economic philosophy that excludes struggling Americans, in the latest in a series of speeches that have ramped up his criticism of those vying to challenge him in the November election.

President Obama's speech at a fundraiser in Illinois came as Republican candidates campaigned in the Midwestern state ahead of a primary vote on Tuesday, the next major test in what has been a drawn-out and chaotic race to choose a Republican presidential nominee.

The Obama camp has stepped into full campaign mode this week, trying to draw a sharper contrast between the president and his divided Republican opponents.

Obama said the conservative field of hopefuls campaigning to take his job would do well to channel the moderation and inclusiveness of Abraham Lincoln, the nation's first Republican president who, like Obama, once represented Illinois in Congress.

"I'm thinking maybe some Lincoln will rub off on them while they're here," Obama said at the fundraiser in Chicago.

Obama said Lincoln - an oft-mentioned Obama role model - understood Americans are one nation, and they rise and fall together. He contrasted that with "on-your-own economics" which he says his Republican foes embrace.

"They've got a simple philosophy: We are better off when everybody is left on their own, everybody writes their own rules," Obama said.

At a fundraiser later in Atlanta, Obama cast the Republican Party as holding a "fundamentally different vision about who we are as a country" and compared his challengers unfavourably with his 2008 presidential opponent, Senator John McCain.

"That shift that has taken place in the Republican Party we haven't seen in a very long time," he said.

"In 2008, the guy I was running against, the Republican nominee, he didn't deny that climate change might be a problem, he thought it was a good idea for us to ban torture, he was on record as having supported immigration reform."

The President's attacks came at a time when his Republican opponents are mired in a polarising primary campaign that has veered into touchy social issues like contraception, and away from the economic problems worrying many Americans.

Mitt Romney, who has struggled to seal his front-runner status, hoped to reclaim his momentum in Illinois, where he is well-organised and expected to do well in the Chicago suburbs.

The former Massachusetts governor spoke on Friday at outside of Chicago, touching on the familiar theme of his campaign: That he is the most qualified candidate because he is an accomplished businessman.

After Illinois, Romney headed to Puerto Rico, which holds its primary on Sunday. The Republicans choose their nominee in a series of primaries and caucuses in US states and territories.

Romney is far ahead in the race for delegates to the Republican National Convention, where the party will formally announce its nominee in August. Yet he has struggled to win over his party's conservative base, which distrusts his past moderate stances on social issues like abortion.

Illinois, where 54 delegates are at stake, has been made more competitive by the surge of former Pennsylvania Senator Rick Santorum, whose popularity with evangelical conservatives could make him appealing in the rural downstate area.

Running a distant third in the primary race is former House Speaker Newt Gingrich, who dismissed Santorum's calls to step aside, insisting that there's probably no circumstance that would lead him to drop out.

Gingrich demanded that Romney stop the barrage of TV attack ads that badly damage him and Santorum each time they seemed poised to make big strides in the race.

He told Romney "to pledge to take every negative ad about every Republican off the air, because it dishonours them, it weakens the Republican Party, and it helps Barack Obama."

Originally published as Obama sharpens criticism of Republicans