Then-Secretary of State Hillary Clinton, left, greets Sen. Elizabeth Warren (D-Mass.) as they arrive for John Kerry’s confirmation hearing before the Senate Foreign Relations Committee in January 2013. (Chip Somodevilla/Getty Images)

The tableau surrounding Hillary Clinton on Monday was impressive: 13 female Democratic senators endorsing the Democratic 2016 presidential front-runner en masse.

The evening fundraising event on Capitol Hill brought in a chunk of campaign cash ahead of an often difficult fundraising month in December. But it was also meant to underscore Clinton’s near-monopoly among Democratic lawmakers who have declared a preference, and her appeal as the first woman with a strong shot at becoming president.

But one particularly influential female Democratic senator didn’t join her colleagues: Sen. Elizabeth Warren (D-Mass.) — the anti-Wall Street crusader who was courted to run as the darling of very liberal Democrats — has not endorsed Clinton, nor has she promised she will.

Her absence served as an awkward reminder of Clinton’s enduring struggle to generate support and enthusiasm among an influential segment of her party’s most liberal members. It also illustrates the leverage that Warren holds in an election that Democrats are calculating will be waged on issues of economic advancement and fairness.

Warren has also declined to endorse Sen. Bernie Sanders (Vt.), Clinton’s main challenger on the Democratic side. When Warren took herself out of the race earlier this year, Sanders was the main beneficiary — inheriting much of the disaffected liberal support that has eluded Clinton.

Democratic presidential candidate Hillary Clinton spoke at a fundraising event where thirteen Democratic senators endorsed her. (Reuters)

“It definitely gives more weight to her endorsement, for whatever she would like to use that weight for,” said Democratic strategist Bill Burton, who was a senior aide in the Barack Obama campaign that defeated Clinton in 2008.

Clinton took the stage to the booming refrain from Katy Perry’s girl-power anthem “Roar” and thanked each of the senators by first name. She drew some of the loudest applause of the night with an emotional pledge to defend abortion rights.

Invoking the killings at a Planned Parenthood clinic in Colorado last week, Clinton said she and the “strong, strong soldiers” on stage with her would defend the organization against Republican attacks.

The event drew about 1,000 supporters who paid between $250 and $2,700 for a ticket. Clinton also held two other fundraising events in Washington on Monday.

“It would take something extraordinary to get all 13 of us here at one time,” said Sen. Barbara Boxer (D-Calif.). “That something extraordinary is Hillary Clinton.”

Each of the senators appearing Monday night with Clinton has previously indicated at least tentative support for her presidential run. Warren, on the other hand, has stayed studiously neutral while pushing the candidates toward more liberal economic policies.

A senior Democratic aide familiar with the planning for the event did not believe that Warren was invited to participate.

Sen. Elizabeth Warren (D-Mass.), left, has yet to endorse Democratic presidential candidate Hillary Clinton. (Pete Marovich for The Post; Spencer Platt/Getty)

Boxer dismissed the suggestion that Warren’s endorsement holdout might indicate deeper tensions among Democrats. “We have 83 percent of the senators supporting Hillary, and it’s wonderful,” she said Monday.

It was unclear what that figure referred to. Thirty-three of 44 Senate Democrats have officially backed Clinton, or 75 percent, while 93 percent of the 14 Democratic women have.

While Warren has clashed at times with the Obama administration — particularly over appointments to the Treasury Department — her relations with Democratic colleagues on Capitol Hill have generally been positive and respectful. One notable exception was last December, when she criticized Democratic leaders for failing to strip a provision weakening the Dodd-Frank financial regulations from a massive spending bill.

But the fact that Warren has not yet endorsed a presidential candidate has been widely interpreted in Democratic circles as an effort to maintain her influence on the campaign agenda rather than as a snub of Clinton. That perception, however, could erode in the coming weeks, especially as more congressional Democrats endorse Clinton. Sen. Sherrod Brown (D-Ohio), who is frequently out front alongside Warren on issues of economic justice, endorsed Clinton in late October.

“It would be notable if Warren were there,” the Democratic aide said of Monday’s event. “I don’t think it’s hugely notable that she is not there.”

A spokeswoman for Warren declined to comment on Monday’s event. The Clinton campaign did not respond to a request for comment.

On Sunday, Clinton went to Warren’s home ground of Boston, where she celebrated the endorsement of the union-backed mayor, Marty Walsh. Warren’s name did not come up.

Before Clinton entered the race, Warren spoke about her in glowing terms and joined all of her fellow female Democratic senators in signing a 2013 letter urging Clinton to run. But with Sanders in the race championing key planks of her economic agenda, Warren has assumed a role above the fray, pushing the candidates to abide by that platform.

Pushed in September about whether she would eventually endorse a candidate, she said, “Right now that’s not where we are.”

Sanders is closest to Warren’s philosophy that the financial system is rigged and that government has been complicit in creating a profoundly unfair balance of power. But Clinton has added a populist edge to her mostly centrist economic platform, telling audiences that the deck is stacked against the middle class and that women, immigrants and the working poor are too often cheated out of their rightful earnings and influence. In a significant policy switch, Clinton also announced her opposition to a massive Asia-Pacific trade deal she had promoted as sec­retary of state in President Obama’s first term.

Two months before the Iowa caucuses, Clinton differs from Warren mostly by degree. Warren favors, among other things, a more level income scale, a higher minimum wage, and tougher rules and enforcement for Wall Street. The scope of that enforcement is the rub, with Warren advocating a breakup of big banks.

Clinton holds a solid lead over Sanders in Iowa after a summer of sliding poll numbers and campaign fumbles.

If Iowa Democrats aren’t head over heels in love, it probably doesn’t matter much, said Peter Brown, assistant director of the Quinnipiac University poll, which conducts monthly surveys in Iowa and New Hampshire.

Despite lower marks than Sanders for handling the economy, Clinton has the support of 51 percent of likely caucus participants, with 42 percent for Sanders in the latest Quinnipiac poll released last week. That means Iowa Democrats are banking that Clinton can win in the general election, Brown said, and suggests that the Warren effect is blunted.

Clinton’s camp resists comparisons with Warren or any suggestion that Clinton takes her cues from the former Harvard professor, who has far less experience in government. Never a Clinton intimate, Warren has made no public attempt to cozy up.

“Hillary’s position is very strong, and I don’t think a process story here or there is going to have much impact on her stance with any particular group in the Democratic orbit,” Burton said.