She later went on to applaud him for all the he was able to accomplish with his campaign and thank his many supporters. "But I believe that, at this moment, our country, our values, and our common vision for a transformed America, are best served by the defeat of Donald Trump and the election of Hillary Clinton", he wrote in a letter posted to Facebook.
"You made a bad situation worse by saying 'I'm going to do whatever they recommend, even though I don't know what the recommendations are, '" said Rep. Jim Jordan, R-Ohio.
Sanders has avoided endorsing Clinton for weeks.
In response to Sanders' endorsement, Clinton said, "I can't help but reflect on how much more enjoyable this election will be now that we are on the same side".
Many Sanders supporters have voiced their opposition to Clinton, refusing to vote for her regardless of her party stance.
Donald Trump's campaign says Bernie Sanders "is now officially part of a rigged system".
"Honestly, I'll make that decision at the convention", Livingston said in an interview Tuesday.
The 74-year-old Vermont senator, who describes himself as a democratic socialist, pledged his "political revolution" which has energised millions of Americans would continue. In a high-profile rally last month, Clinton was embraced by Massachusetts Sen. It's not just about getting Elizabeth Warren on stage and all these progressive leaders on stage, including Bernie Sanders.
Sanders held out about a month past the ending of the primary season. When Sen. Jeanne Shaheen, a Clinton supporter, told the audience, "We need to elect Hillary", some Bernie supporters stood and shouted, "No", which was followed by chants of "Hillary" in the crowd. He will endorse her today - fans angry! Sanders himself penned a heartfelt letter to supporters, acknowledging that he understands some of them may be "disappointed".
"I am proud to be fighting alongside you", she said.
Andy Mager, one of the Syracuse for Sanders organizers, said he also liked what he heard from Clinton on Tuesday.
The White House rivals appeared together at a rally in New Hampshire where the veteran senator offered his congratulations.