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

With a series primary votes on Tuesday that could see Clinton all but seal the party nomination, the White House refused to rule out an endorsement within days.

"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 the White House has publicly been wary of putting Obama's hand on the scale during a surprisingly long and sometimes bitter primary race against leftist Bernie Sanders.

White House officials have been wary about alienating millions of young voters who have flocked to Sanders' strident campaign, which has given voice to popular anger about income inequality.