WASHINGTON—A big question mark remains on the list of U.S. presidential hopefuls – will Vice President Joe Biden enter the race on the Democratic side, joining former Secretary of State Hillary Clinton and Senator Bernie Sanders? The answer could come any day now and put an end to weeks of speculation and intrigue in Washington and beyond.

Biden has an extensive political resume: vice president for nearly seven years after 36 years as a U.S. senator. Plain-spoken and long-winded, Joe Biden is no stranger to the campaign trail.

“We will not go back to the 50s in social policy, to the Cold War in our foreign policy, or to the policies of the last administration in our economic policy,” Biden said.

But Biden isn’t saying whether he will mount his third campaign for president. The possibility seemed remote months ago after the death of his son, Beau, and as Hillary Clinton dominated the Democratic field. But with Clinton now mired in controversy over her use of a private email account as secretary of state, and with Senator Sanders drawing enthusiastic crowds, some political observers see an opening for Biden should Clinton stumble further.

President Barack Obama is staying neutral.

“What I would say is that both Joe and Hillary are wonderful people, great friends. Joe’s been as good a vice president as I think we’ve seen in American history, been at my side in every tough decision I’ve made,” said Obama. "Hillary Clinton was one of our best secretaries of state.”

And Clinton is choosing her words carefully, too. “I just want him to reach whatever he thinks the right decision is,” she said.

But some Republican presidential contenders are weighing in.

“I think he (Biden) sees a vulnerability in Secretary Clinton," said senator Lindsey Graham. 'So I don’t know what Joe’s going to do, but if he’s ever wanted to run, now’s his best chance.”

Most presidential aspirants of both parties laid the groundwork for their bids a year or more ago and have already spent months on the campaign trial. Should Biden run, he would have to play catch-up in forming – and financing – a campaign.