Clinton struck a note Tuesday with Livingston and other Sanders supporters when she talked about encouraging younger people with progressive values to run for political office.
Sanders added he wanted to make it as clear as possible why he has endorsed Clinton, "and why she must become our next president".
Livingston was a leader of the Syracuse for Sanders presidential primary campaign, helping Sanders capture about 49 percent of the vote in Central New York's 24th Congressional District.
"Pennsylvanians must do everything we can to defeat Donald Trump in November and ensure he doesn't get anywhere close to the White House", she said. Sanders thanked his backers for their support during his campaign. "That's what democracy is about".
While Clinton took only a few days to endorse Obama after the primaries in 2008, Sanders has held out for the past month, seeking to influence the party's platform and future.
Sanders is joining Clinton at Portsmouth High School to discuss their commitment to building an America that is stronger together, according to a Clinton campaign release, but the self-proclaimed Democratic socialist is expected to formally endorse Clinton at the event.
Clinton was the target of a Snapchat filter when conservative group Judicial Watch sponsored one on the day of her Benghazi testimony last October. He responded, "Because I haven't heard her say the things I think need to be said". "But I am happy to tell you that at the Democratic Platform Committee, which ended Sunday night in Orlando, there was a significant coming together between the two campaigns, and we produced, by far, the most progressive platform in the history of the Democratic Party".
Clinton said she supported bias training and nationwide use-of-force guidelines for police officers to curb the "tragedy of black men and women and black children being killed in police incidents".
Supporters couldn't help but point out Sanders' hypocrisy.
"Just watching her speech after Bernie was done, everything she said today sounded like what Bernie was saying", Livingston said.
Sanders began by reminding his supporters that the "political revolution" was never meant to be about a single candidate or election cycle - and that the revolution's next strategic priority must be to elect Hillary Clinton.
"I would do so without great enthusiasm because she is extremely tainted", Mager said.
Trump, who has proclaimed himself "the law and order candidate" amid rising gun violence, will campaign in IN later Tuesday.