04/09/2012 12:49 EDT | Updated 06/09/2012 05:12 EDT

Gingrich concedes Romney 'likely' Republican nominee

Rival Newt Gingrich may have conceded frontrunner Mitt Romney will "most likely" win the Republican presidential nomination, but the former governor faces several challenges in pivoting his campaign to take on the Democratic opponent — U.S. President Barack Obama.

Chief among them is fundraising, with the incumbent Obama widely believed to have a significant advantage in bringing in money through an established network of donors that propelled him to victory four years ago.

After sweeping last week's primaries in Wisconsin, Maryland and Washington D.C., Romney is looking to solidify his status as the all-but-certain nominee in several state contests later this month, while also focusing on Obama's record in office —instead of targeting his Republican challengers.

Romney, a former Massachusetts governor and CEO of an investment firm, has emphasized his business background in his pitch to convince voters he is best equipped to handle the country's economic recovery. He has accused Obama of bringing in policies that hindered economic growth through over-regulation.

Last week also saw Romney pick up endorsements from high-profile Republicans such as Marco Rubio and Paul Ryan. Both are rumoured to be possible vice-presidential candidates on the party's ticket in November.

But Romney has struggled to generate enthusiasm for his campaign and connect with voters, especially among the party's religious conservatives. Many among the party's right-wing grassroots view with suspicion Romney's varying positions on issues such as abortion and gun control.

Santorum visits ailing daughter

Meanwhile, Romney's main rival Rick Santorum, a former senator and staunch social conservative, continues to dismiss growing pressure from Republican circles to bow out of the race for the sake of party unity.

Santorum is vowing to stay in at least until the April 24 primary in his home state of Pennsylvania, where he hopes to score a win that could rejuvenate his upstart campaign against the heavily favoured and better-financed Romney juggernaut.

The candidate took a break from campaigning in Pennsylvania during the Easter long weekend and cancelled scheduled appearances on Monday to be with his ailing three-year-old daughter, Bella, who has a severe genetic disorder.

While Santorum's campaign said his daughter's condition was improving, the Romney camp said it was suspending ads critical of Santorum in the state for the day out of respect for the family.

Santorum spokesman Hogan Gidley insisted the campaign "moves on" and has a "full slate" of events Tuesday in the state. However, the candidate himself all but said a loss in Pennsylvania would end his bid.

"We have to win here," Santorum told reporters in the Pittsburgh area last week.

Presidential bid 'much harder than I thought': Gingrich

On Sunday, Gingrich acknowledged Romney would be "far and away the most likely" presidential nominee, but added he would stay in the race to "influence the party's platform."

Gingrich was once viewed as Romney's main challenger after a surprise South Carolina win in January, only to see his campaign fortunes and fundraising stall in the face of his rival's continued victories and accumulation of party delegates.

Gingrich, known for his confident declarations, admitted in a Fox News Sunday interview that running for president "turned out to be much harder than I thought it would be."

"I do think there's a desire for a more idea-oriented Republican Party, but that doesn't translate necessarily to being able to take on the Romney machine," Gingrich said.

"It turned out, we didn't have anything like his capacity to raise money."

The fourth nominee, Ron Paul, enjoys the backing of a grassroots movement of fervent supporters and donors that has now surpassed Gingrich's financial and volunteer support.

Paul, a Texas congressman and outspoken libertarian who has repeatedly called for a limited government and dramatic reductions in federal spending, has long been rumoured to be a possible third-party challenger to Obama and the Republican candidate.