By Arun Kumar 

Washington, Mar 18 (IANS): As Hillary Clinton and Donald Trump surged forward in the US presidential race, President Barack Obama reportedly threw his weight behind the former, while the Republican establishment intensified efforts to dump the brash billionaire.

In unusually candid remarks, Obama privately told a group of Democratic donors last Friday that time is coming to unite behind his 2008 rival, the New York Times reported.

The President, according to the Times, told the group in Austin Texas that Clinton's rival Bernie Sanders was nearing the point at which his campaign would end, and that the party must soon come together to back her.

"Obama acknowledged that Clinton was perceived to have weaknesses as a candidate, and that some Democrats did not view her as authentic," it said.

"But he played down the importance of authenticity, noting that President George W. Bush - whose record he ran aggressively against in 2008 - was once praised for his authenticity," the Times said.

White House Press Secretary Josh Earnest denied Thursday that Obama had endorsed Clinton, but acknowledged that he had made a case for party unity.

The Democrats, Obama told them "need to be mindful of the fact that our success in November in electing a Democratic President will depend on the commitment and ability of the Democratic Party to come together behind our nominee."

"And the President did not indicate or specify a preference in the race," Earnest insisted noting that Obama had praised both Clinton and Sanders.

"But once this (primary) process comes to a conclusion, everybody in the Democratic Party will understand the stakes of the debate, and given those stakes, will need to unify behind the Democratic Party nominee to ensure that he or she can win in November," he said.

The Washington Post said Obama and his top aides have been strategising for weeks about how they can reprise his successful 2008 and 2012 approaches to help elect a Democrat to replace him.

Thus out of concern that a Republican president in 2017 would weaken or reverse some of his landmark policies, Obama "is poised to be the most active sitting president on the campaign trail in decades," it said.

Meanwhile, on the Republican side, prominent conservatives called for a unity ticket and a convention fight to stop in "a sign of the growing desperation in the party establishment to find an alternative to the billionaire businessman," CNN reported.

Conservatives gathered in Washington Thursday to discuss ways to thwart Trump's march to the nomination.

One proposal included a unity ticket involving Trump's closest rival Ted Cruz and Ohio Governor John Kasich, CNN said citing a source familiar with the conversation.

But the group decided not to commit to that pairing "because of the egos involved," it said.

It also left the door open to potentially supporting a third party race if Republicans are unable to stop Trump.