Elizabeth Warren Only Female Democratic Senator Missing From Hillary Clinton Endorsement Event

WASHINGTON -- One by one tonight, 13 female Democratic senators took the stage inside the packed ballroom at the Hyatt Regency in Washington, DC to raise money and announce their support for Hillary Clinton.

But it was the person missing from the group who stood out most.

Elizabeth Warren, the progressive lawmaker from Massachusetts, was the only female Democratic senator who wasn't at the endorsement event and fundraiser for Clinton's presidential campaign.

Warren, who many thought would run for president herself, has so far declined to back anyone in the primary. She is also one of only six Democratic senators who has not endorsed Clinton.

Clinton's campaign said it invited the 13 female senators who have endorsed Clinton to attend tonight's event, but would not say whether or not they invited Warren. They also would not comment about Warren's absence.

"We're honored to have 13 women Senators coming together to endorse and support Hillary Clinton," Clinton spokeswoman Christina Reynolds said in a statement, "This is a sign of the broad support Clinton is receiving from women across the country who know she'll fight for us."

Warren's senate office declined to comment.

Last year, all of the female Democratic senators -- including Warren -- signed a secret letter to Clinton encouraging her to run for president. Warren later clarified that while she supports Clinton running, she had not endorsed her.

"You know, all of the women -- Democratic women, I should say, of the Senate -- urged Hillary Clinton to run, and I hope she does," Warren told ABC News' George Stephanopoulos last year when asked if she would back Clinton.

"Hillary is terrific," she replied when asked again.

Many of the other senators who signed that letter were at the event tonight, including Senators Tammy Baldwin, Barbara Boxer, Maria Cantwell, Dianne Feinstein, Kirsten Gillibrand, Heidi Heitkamp, Mazie Hirono, Amy Klobuchar, Claire McCaskill, Barbara Mikulski, Patty Murray, Jeanne Shaheen and Debbie Stabenow.

No one, however, made any mention of Warren's absence during the program. "I'm not sure why she wasn't," Mikulski said coyly, "Maybe she has a cold."

The fundraiser tonight, which was held just blocks from Capitol Hill, was only the second of Clinton's fundraisers to be open to the press. More than 1,000 people attended and tickets ranged from $250-$2,700, according to the campaign.

Clinton, who spoke following remarks from each of the Senators, spoke broadly on a range of issues such as equal pay, paid leave, marriage equality, climate change, the economy and terrorism.

She also offered her thanks and praise of her former colleagues.

"They have so much courage and smarts, the combination of grit and grace," Clinton, a former senator, said pointing to the women standing on stage behind her, "It was the honor and privilege of my life to serve with them."