WATCH: White House Praises Biden Among Speculation He Could Challenge Hillary in 2016

'The president has indicated that his view that the decision that he made, I guess 7 years ago now, to add Joe Biden to the ticket as his running mate was the smartest decision that he has ever made in politics.'

WASHINGTON - President Barack Obama and Vice President Joe Biden have a lot to catch up on.

Just back from two weeks of vacation on the tony Massachusetts island of Martha's Vineyard, Obama is holding his weekly lunch with Biden Monday. While the president has been away, Biden has been ramping up discussions about whether he will seek the Democratic presidential nomination in 2016.

If Biden runs, it would put Obama in an awkward position. Hillary Rodham Clinton, his former secretary of state, is currently the front-runner in the Democratic primary, but she's been plagued with questions about her use of personal email and a private server while in government.

The White House on Monday left open the possibility that President Barack Obama would make an endorsement in the 2016 Democratic presidential primary race, which could feature his vice president running against his former secretary of state.

Obama is close to both leaders and has not indicated support for either in a potential match-up.

White House spokesman Josh Earnest, asked whether the president would openly back one of the Democratic candidates, said it was not out of the question.

"I wouldn't rule out the possibility of ... an endorsement in the Democratic primary," Earnest told reporters, adding that Obama planned to vote in the primary election in Illinois, the state he represented as a senator before entering the White House.

Earnest repeated that Obama has said choosing Biden as his running mate in 2008 was the best political decision he had ever made.

He said Biden, who has already been on a national political ticket twice, knows the challenges that would face him if he decided at this stage to run.

"There is probably no one in American politics today who has a better understanding of exactly what is required to mount a successful national presidential campaign," Earnest said.

Despite her struggles, Clinton remains the prohibitive favorite, not least because of an early start on fundraising and an advanced get-out-the-vote structure in early voting states.

A match-up between the two would no doubt create awkwardness for the president and others in the White House whose priority is to elect another Democrat in 2016 to preserve the record established under Obama's watch.