With months of the campaign past, 40 of the 46 of senators who caucus with the Democrats have endorsed Clinton.
Here is the latest list of senators backing Clinton, who herself served in the chamber for eight years, starting in 2001:
Sen. Richard Blumenthal of Connecticut -- Told CNN at a 2014 University of Connecticut event that he "would support her when and if she" runs.
Sen. Cory Booker of New Jersey -- Booker gushed about Clinton's qualifications in an NBC News interview. "There are few candidates in history who are as qualified or ready for the job of president as Hillary Clinton. I'm excited about her candidacy and her vision for our country," Booker said.
Sen. Barbara Boxer of California -- Signed a letter with all other female senators backing a Clinton run.
Sen. Sherrod Brown of Ohio -- Endorsed Clinton in late October, telling reporters in Ohio that she is the best person to keep the state moving forward.
Sen. Maria Cantwell of Washington -- Signed a letter with all other female senators backing a Clinton run.
Sen. Ben Cardin of Maryland -- Headlined a December 2014 fundraiser organized by Ready for Hillary.
Sen. Thomas Carper of Delaware -- After homestate favorite Vice President Joe Biden declined to run, Carper issued a statement endorsing Clinton. "Nearly 15 years ago, Hillary and I were elected to the United States Senate together, and since then I've had the joy and privilege of wishing my friend a happy birthday each year on October 26th. When I called her today, I had the opportunity to deliver a unique birthday message -- that I am proud to endorse her as the next Democratic nominee for President," he said.
Sen. Bob Casey of Pennsylvania -- At a 2014 event in Philadelphia, Casey told CNN that it was too early to talk about Clinton 2016. "I'm not going to get into that," he said. But after Clinton announced her run, he emailed supporters to say, "Having served with her in the Senate, I know she has always been a strong advocate for the middle class and I'm confident she will work tirelessly to ensure that Pennsylvania families have the chance to get ahead and stay ahead."
Sen. Chris Coons of Delaware -- Announced on CNN's "New Day" on November 10 that he was backing Clinton, several weeks after Biden announced he wasn't running.
Sen. Joe Donnelly of Indiana -- Said in a November 13, 2015 statement "Hoosiers deserve and advocate for middle- and working-class families in their next president."
Sen. Dick Durbin of Illinois -- Headlined a June 2014 fundraiser organized by Ready for Hillary.
Sen. Dianne Feinstein of California -- Signed a letter with all other female senators backing a Clinton run.
Sen. Al Franken of Minnesota -- The senator told MSNBC this in December 2014: "I think that I'm ready for Hillary. I think that we've not had someone this experienced, this tough, and she's very, very impressive." Franken has also expressed support through Ready for Hillary.
Sen. Kirsten Gillibrand of New York -- Signed a letter with all other female senators backing a Clinton run.
Sen. Martin Heinrich of New Mexico -- Headlined a July 2014 fundraiser organized by Ready for Hillary.
Sen. Heidi Heitkamp of North Dakota -- Signed a letter with all other female senators backing a Clinton run.
Sen. Mazie Hirono of Hawaii -- Signed a letter with all other female senators backing a Clinton run.
Sen. Bill Nelson of Florida -- Nelson reportedly called Clinton in December 2014 to urge her to run. "It's time for a woman," he told The Tampa Bay Times. "I'm all for Hillary."
Sen. Tim Kaine of Virginia -- Headlined a May 2014 fundraiser organized by Ready for Hillary.
Sen. Amy Klobuchar of Minnesota -- Signed a letter with all other female senators backing a Clinton run.
Sen. Patrick Leahy of Vermont -- Told Vermont Public Radio this in June 2014: "I told her if she decided to run I would support her and would be willing to do whatever she likes. I've made no secret of that ever since then."
Sen. Joe Manchin of West Virginia -- He told Politico in January 2014 that he wants Clinton to run. "I don't know if there's anyone more qualified. I've seen it all," he said.
Sen. Ed Markey of Massachusetts -- Markey endorsed Clinton in a statement to reporters in October 2015. "I believe there is no one better to lead the effort to combat climate change here at home against the climate deniers and around the world as we partner with other nations to implement clean energy solutions," he said.
Sen. Claire McCaskill of Missouri -- Signed a letter with all other female senators backing a Clinton run.
Sen. Barbara Mikulski of Maryland -- Signed a letter with all other female senators backing a Clinton run.
Sen. Patty Murray of Washington -- Signed a letter with all other female senators backing a Clinton run.
Sen. Chris Murphy of Connecticut -- Murphy endorsed Clinton for president in June 2015. "I am proud to support her. I'm all in," he said in a statement.
Sen. Gary Peters of Michigan -- Peters tweeted in April, "I'm standing with @HillaryClinton because she's the middle class champion that America needs."
Sen. Jack Reed of Rhode Island --
Formally endorsed Clinton in April 2016
Sen. Harry Reid of Nevada -
Formally endorsed Hillary Clinton in an interview
with CNN in February.
Sen. Brian Schatz of Hawaii -- Tweeted Monday morning that he was backing Clinton in 2016.
Sen. Charles Schumer of New York -- The outspoken senator has attended a number of Clinton events, including fundraisers."
Sen. Jeanne Shaheen of New Hampshire -- Signed a letter with all other female senators backing a Clinton run.
Sen. Debbie Stabenow of Michigan -- Signed a letter with all other female senators backing a Clinton run.
Sen. Jon Tester of Montana -- Formally endorsed Clinton on June 17
Tom Udall of New Mexico -- Udall tweeted in April that he was "ready for Hillary." Clinton, he wrote, "has the experience to be our next President."
Sen. Mark Warner of Virginia -- Headlined a July 2014 fundraiser organized by Ready for Hillary.
Sen. Sheldon Whitehouse of Rhode Island -- Told The Hill in January 2014 that he was backing Clinton.
Sen. Ron Wyden of Oregon -
Clinton in January.
Not all senators, however, are ready to endorse Clinton and some have been unclear about their support.
Sen. Angus King of Maine (Independent who caucuses with Democrats).
Sen. Robert Menendez of New Jersey.
Sen. Elizabeth Warren of Massachusetts --
She signed a letter with all other female senators backing a Clinton run but as recently as last month, she still has declined to make a public, formal endorsement
HIS OWN SUPPORTER
Only two senators who caucus with Democrats have outright lined up against Clinton -- and one of them is Bernie Sanders of Vermont, her rival for the party's nomination. And on April 13, Sen. Jeff Merkley of Oregon became the first of Sanders' Senate colleagues to formally endorse him.