(CNN) Hillary Clinton and Bernie Sanders are blitzing California on an intense final day of campaigning in the 2016 Democratic primary.

The former secretary of state on Tuesday is expected to become the first female presumptive nominee of a major party -- a feat that will likely raise pressure on Sanders to drop his bid quickly. The Vermont senator has been loathe to discuss exiting the race -- even raising the potential over the weekend of a contested convention -- but struck a more subdued note Monday.

"Let me just talk to you after the primary here in California, where we hope to win," Sanders told reporters at a news conference. "Let's assess where we are after tomorrow before we make statements based on speculation."

He didn't once mention the word "Philadelphia" -- home of this year's Democratic convention -- during the news conference.

But amid clear signs that Democratic Party grandees are moving to shut down the campaign after Tuesday, a party source told CNN that President Barack Obama spoke to Sanders by phone on Tuesday. Two other Democratic sources, meanwhile, said the President is poised to deliver his endorsement of Clinton as early as this week.

For her part, Clinton focused on presumptive Republican nominee Donald Trump, who is under fire for repeatedly accusing a judge overseeing a lawsuit involving Trump University of bias because of his Mexican heritage. Trump added to the controversy Sunday by saying he would have similar concerns if a Muslim judge supervised the case.

"I'm waiting for him to say because of all the bigoted things he has said about women that a woman judge couldn't preside," Clinton told a lunchtime crowd near Los Angeles. "By the time he's finished, nobody's going to be left in this country that he is going to have exempted from insults."

The Clinton-Sanders battle is playing out far longer than most would have predicted at the beginning of the campaign season. With his critique of economic inequality, Sanders, a 74-year-old self-described democratic socialist, has become the unlikely hero of the young, progressive Democratic base. Still, Clinton is on the verge of the nomination after she dominated contests in the South and won large states including New York and Pennsylvania.

Clinton's victory in Sunday's Puerto Rico primary left her just 26 delegates shy of being declared the presumptive Democratic nominee, a milestone she will easily clear in the contests playing out in six states Tuesday. Clinton is likely to pass the magic number of 2,383 delegates soon after the polls close in New Jersey. That means that California's primary may serve as a litmus test of the party's interest in Sanders' liberal policies -- even if it won't sway the outcome of the nomination.

'Not over until it is over'

In her own question-and-answer session with reporters Monday, Clinton pointed out that Tuesday would mark exactly eight years since she folded her 2008 primary campaign and endorsed Barack Obama, in a strong hint that Sanders should do the same.

"It is not over until it is over and tomorrow is a really important day," Clinton told CNN's Dan Merica in Compton, California, saying she wanted for now not to focus on the historic nature of her candidacy because she wanted to ensure everyone came out to vote.

Asked how she could change Sanders' mind about fighting on to the convention, Clinton said she would take stock after Tuesday but vowed to work hard to unify the party to defeat Trump.

"I certainly am going be reaching out to Senator Sanders and hope he will join me in that because we have got to be unified going into the convention and coming out of the convention to take on Donald Trump."

A musical finish

The Vermont senator is closing out the primary season with one big final event planned later Monday: a get-out-the-vote concert in San Francisco featuring Dave Matthews, Fantastic Negrito, Fishbone and actor Danny Glover. Clinton will counter her own musical event starring Christina Aguilera, Andra Day, John Legend, Ricky Martin, and Stevie Wonder.

Once the results are in Tuesday night, the Clinton campaign will move quickly to bring Democrats together after a bruising campaign. Clinton said in an interview with CNN's Jake Tapper on Friday that she would begin a concerted push to unite Democrats beginning on Wednesday. CNN's Jeff Zeleny reported Monday that the effort could include an early -- and highly significant -- endorsement from Obama.

Obama, who remains highly popular among Democrats and will play a key role in uniting the party, has held off on an endorsement so far as Clinton and Sanders have battled it out. But Zeleny, citing two well-placed Democrats, said the Clinton campaign is working with the White House to coordinate the unity push, and comments by the President could make the pressure on the Vermont senator to quit insurmountable.

White House spokesman Josh Earnest said that he didn't have any announcements to make as yet about a Presidential endorsement. Pressed by CNN's Michelle Kosinski whether Obama would indeed endorse someone on Wednesday, he answered: "I don't know. Maybe he will."

Earnest acknowledged Tuesday that media organizations, including CNN, have now called Clinton the presumptive nominee, but added that the most important "super delegate" -- Obama -- is not prepared to make a public endorsement "at this point."

"However at this point there is at least one super delegate. The one who works in the oval office who is not prepared to make a public declaration about his endorsement at this point. But stay tuned and we'll keep you updated," Earnest said at the White House briefing.

Earnest also shot down the possibility of a meeting with Obama and Clinton tomorrow while the President is in New York.

Tight race in California

Several recent polls in California have shown a close race between Clinton and Sanders within the margin of error. Sanders is banking on the enthusiasm of young, more liberal voters while Clinton is relying heavily on minority communities and may be hoping that the largely positive reviews among Democrats of her speech last week lacerating Donald Trump as unfit for the presidency will give her a late boost among undecided voters.

Sanders hopes that a victory in California would bolster his claims that over the long Democratic primary, the party has turned against Clinton and lacking her political baggage, he'd be a much stronger candidate to battle Trump in November.

His strategy requires convincing Democratic superdelegates -- party officials and others who are free to vote however they wish at the convention -- to vote for him instead in Philadelphia.

Sanders vowed as recently as Saturday to contest the Democratic convention. Asked by reporters Sunday if that was still his position, he simply responded: "Absolutely."

And he intensified his attacks on Clinton on CNN's "State of the Union," saying he is bothered by the potential conflict-of-interest of the Clinton Foundation's acceptance of gifts from foreign governments during her tenure as secretary of state and said Clinton's backing of interventions in Iraq and Libya proved she was too hawkish.

But the Sanders approach is a long shot not just because most superdelegates prefer Clinton. It would require Sanders to convince his party that superdelegates should cancel out the verdict of Democratic primary voters themselves, since Clinton has several million more voters in the primary than her rival and has a lead of 300 pledged delegates allocated after state primaries and caucuses throughout the entire Democratic campaign.

"The fact that she will have won by millions of votes, the popular vote, will have a majority of the pledged delegates, will mean that the superdelegates that are committed to her will remain committed to her," California Rep. Adam Schiff, a Clinton supporter told CNN on Monday.