The expected Obama endorsement, reported by a premier news agencies, would come as a welcome boost to Clinton and to Democrats concerned that the party needs to turn its attention fully to campaigning against Republican nominee Donald Trump.
While he has made remarks indicating a preference for Clinton, his former secretary of state, Obama has so far avoided a clear endorsement and has focused his remarks about the campaign on blasting Trump.
A senior White House official would not comment on timing of any endorsement but said Obama is eager to campaign where he might be useful.
Clinton has long been the front-runner to be the Democratic nominee in the November 8 election but has faced an unexpectedly tough fight against Sanders, a US senator from Vermont who has attacked her from the left.
She is expected to clinch the party nomination today when voters in six states cast ballots. Her campaign hopes an expected victory in New Jersey will give her enough delegates to effectively lock up the nomination early in the evening, before the results come in from California, the biggest electoral prize and likely the last to report results.
But although he lags well behind in delegates needed to win the nomination, Sanders has vowed to take the fight to the Democratic National Convention in July.
Clinton supporters said on Monday that Sanders should acknowledge her victory and move toward uniting the party behind her.
"I think at the bare minimum what he can do is not try to delegitimize the process or call into question the fact that Hillary Clinton is truly the nominee after Tuesday," Brian Fallon, Clinton's press secretary, said.
Mo Elleithee, executive director of Georgetown University's Institute of Politics and Public Service and a former Clinton campaign staffer, said her June 2008 concession to Obama in the closely fought Democratic primary that year should be a roadmap for Sanders.
"She gave her supporters who wanted a chance to vote for her a chance to vote for her. Then she stood up and said to her supporters, 'Now let's get behind the presumptive nominee,'" Elleithee said.
Clinton has 2,357 delegates going into today's contests, just 26 short of the 2,383 she needs to clinch the nomination at next month's convention in Philadelphia.
Montana, North Dakota, South Dakota and New Mexico also hold nominating contests today, but most attention will focus on California. Clinton once held a sizable lead there over Sanders, but opinion polls in recent days show a dead heat between the two.
While Clinton expects to become the presumptive nominee regardless of the California result, a Sanders victory there could embolden his supporters to urge him to wage a fractious convention fight.
It could also help Trump argue that she is a weak candidate. Trump secured the delegates he needed to clinch the Republican nomination last month, leaving him free to focus on battering Clinton.
Sanders' campaign appeared to burn through cash to get to the final nominating contests, ending April with just $5.8 million on hand, compared to Clinton's $30 million. The senator has not released his May fundraising figures. Spokesman Michael Briggs said in an email the campaign was "doing fine."
Clinton heads into Tuesday's contests with a victory this weekend in Puerto Rico's primary. And though Sanders could campaign in Washington, DC, ahead of the final primary of the year on June 14, Clinton is expected to win there.

Trump, a real estate developer, has regularly stirred up controversy on the campaign trail and has frequently dismayed Republican establishment leaders. In recent days, his comments about a judge he believes to be biased against him because he is Mexican-American have drawn a fresh wave of criticism, including concern in his own party. On Monday Trump rejected criticism of his allegations of bias, insisting his concerns were valid.

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