Rep. Nancy Pelosi (D-Calif.) is the last of the holdouts.

The House minority leader has dropped plenty of hints that she wants Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE to prevail in her historic run for the White House, but she’s withheld an official endorsement amid the primary contest pitting Clinton against Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.), a liberal icon.

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With Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE's Thursday endorsement of Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE, Pelosi is now the only top congressional leader of either party, in either chamber, to withhold an endorsement this year.

The silence has become more intriguing in the weeks leading up to Tuesday's primary in her home state of California, but Pelosi has hinted she doesn't want her endorsement to undermine voter turnout in the state. True to form, she's remained non-committal.

“I will make an endorsement, and I’ll decide when that is,” she told reporters in San Francisco last month. “But I won’t be telling you that right now.”

Pelosi's reticence highlights the tightrope she's walking in her different roles as a party leader, campaign strategist, monster fundraiser, California legislator, liberal advocate and feminist champion.

It also underscores the tricky political environment Democratic leaders are navigating in the face of Sanders’s resounding success.

The Vermont senator has drawn an enormous following few foresaw a year ago, and his thundering promotion of economic justice has reverberated with millennials, blue-collar workers and otherwise disenchanted voters both distrustful of Clinton and compelled by the thought of sending an outsider to the White House.

Many Sanders supporters are already furious at the Democratic Party for its handling of the primary, accusing top leaders –– most notably Rep. Debbie Wasserman Schultz (Fla.), who heads the Democratic National Committee –– of rigging the contest in favor of Clinton. And Pelosi's endorsement of the former secretary of State might only fuel that outrage and alienate Sanders's backers to the detriment of the party in November.

Instead, Pelosi has trod delicately around those divisions by praising both Wasserman Schultz and Sanders, while emphasizing the need to channel the energy Sanders has generated by getting his supporters to the polls –– even if he's not on the ticket.

Last month, Pelosi called him “a positive force” for Democrats up and down the ballot.

“He has awakened in some people an interest in the political process that wasn't there,” she said.

Still, Pelosi has also left little mystery about which Democrat she favors.

She's repeatedly characterized Clinton as among the best prepared presidential candidates in the nation's history; she often couches her evasive predictions about the next White House resident with an unsubtle, “Whoever she may be;” and she's done nothing to disguise her excitement in the thought of electing the country's first woman as commander in chief.

John Hudak, congressional expert at the Brookings Institution, said Pelosi has withheld her official endorsement both because she takes pride in her role as “a peacemaker” atop the party, and “for fear of being accused of playing favorites or trying to influence the vote.”

“She's … been around politics long enough to know that the energy around the Sanders campaign could be very positive for the party, or very negative,” Hudak said by phone Friday. “She's going to work very hard to bring progressives into the fold.”

Pelosi is hardly the only prominent Democrat to remain officially neutral. President Obama, Vice President Biden, former Vice President Al Gore Albert (Al) Arnold GoreFox's Napolitano: 2000 election will look like 'child's play' compared to 2020 legal battles Who calls an election? Why we need patience and nonpartisanship this time Universal mail-in voting jeopardizes the equal right to vote, but absentee voting protects it MORE and Sen. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (D-Mass.), another liberal paragon, have also declined to endorse through the primary.

But Pelosi's position sets her apart from every other House Democratic leader –– including Reps. Steny Hoyer (Md.), Jim Clyburn (S.C.), Xavier Becerra Xavier BecerraOVERNIGHT ENERGY: Trump casts doubt on climate change science during briefing on wildfires | Biden attacks Trump's climate record amid Western wildfires, lays out his plan | 20 states sue EPA over methane emissions standards rollback 20 states sue EPA over methane emissions standards rollback Investigation underway after bags of mail found dumped in Los Angeles-area parking lot MORE (Calif.) and Joe Crowley (N.Y.) –– all of whom are backing Clinton both in voice and, increasingly, on the campaign stump.

Other Democrat leaders have adopted competing strategies when it comes to timing their endorsements around their home-state primaries. Clyburn, for instance, endorsed Clinton just a few weeks before voters went to the polls in South Carolina in February. That same month, Senate Minority Leader Harry Reid Harry Mason ReidSenate Republicans signal openness to working with Biden Mellman: The likely voter sham Bottom line MORE (D-Nev.) waited a few days after Clinton won Nevada’s primary to throw his weight behind her.

Pelosi never made an official endorsement in the 2008 primary contest between Clinton and Obama, then an Illinois senator. But she caused a minor uproar when she amplified her opposition to the Democrats' super-delegate system –– a position she's held for decades –– at a point when Clinton was ahead, outraging some voices in the Clinton camp, who saw it as a tacit endorsement of Obama.

In February, Pelosi told The Hill she'd switch gears this time around and back one of the Democratic primary contenders. She didn't say when, but she made no bones about predicting the eventual nominee.

“I assume," she said at the time, "it will be Hillary Clinton.”