Obama poised to endorse Hillary Clinton

President Barack Obama edged closer to a formal endorsement of Hillary Clinton as the Democratic presidential nominee Monday, with his White House suggesting the next 48 hours would be pivotal.

With a series of key primary votes on Tuesday that could see Clinton all but seal the party nomination, the White House would not rule out an endorsement by Wednesday.

"Once the voters in New Jersey and California have an opportunity to express their preference," said White House spokesman Josh Earnest, "then we may be in a position where we have a much greater sense of what the outcome is likely to be."

It is an open secret in Washington that President Barack Obama intends to endorse Clinton.

But he wants to avoid alienating millions of young voters who have flocked to Bernie Sanders's energetic campaign. Sanders has won 10 million votes during the primaries, three million fewer than Clinton.

Obama's recent high poll ratings all but secure him a central role in the effort to secure a third White House term for Democrats.

According to Gallup, his approval is now 51 percent, almost double George W Bush's approval at the same point in his presidency.

Polls show Obama remains hugely popular among college educated women, who could prove vital in states like Virginia and Pennsylvania. He is also popular among black, Hispanic and Asian voters.