Bernie Sanders is expected to officially endorse his former rival next week.
Both campaign teams are reported to be in talks about a possible event to make the announcement in New Hampshire.
Mr Sanders said he would vote for Ms Clinton in November, but he has withheld official endorsement as he attempted to negotiate policy details with her campaign.
If the event goes ahead next week, it would come after a committee within the democratic party makes its final votes on Ms Clinton's policy document, which would be presented at the national convention in late July.
The endorsement from Mr Sanders looks more likely after Ms Clinton revealed her plan to scrap tuition fees for students attending public in-state colleges and whose parents earn less than $125,000 per year.
In an interview with MSNBC on Wednesday, Mr Sanders said: “I think at the end of the day, there is going to be a coming together, and we're going to go forward together and not only defeat Trump, but defeat him badly.”
He said the pair "came together on higher education".
The interviewer pressed the Democrat as to whether there might be an endorsement of Ms Clinton.
“That’s correct,” Mr Sanders replied.
In recent weeks the Vermont senator has tweeted support for Ms Clinton on her plans to eliminate college tuition fees, and has also applauded her “progressive” agenda on implementing Wall Street reforms, expand Social Security and to reject money from lobby groups, known as super PACs.
The shadow hanging over Ms Clinton relating to the misuse of her personal email server has lightened this week as it was announced she will not face any criminal charges from the Justice Department following FBI recommendations.
Mr Sanders famously quipped during a debate in October that the American public “are sick and tired of hearing about [her] damn emails”.
Mr Sanders, who is still surrounded by a large entourage and security staff as he recently returned to the senate, said before the final democratic primary in Washington DC that he was willing to team up with Ms Clinton to defeat Donald Trump.
He insisted that he would not drop out of the race before 14 June, the date of the last primary, because he wanted to ensure that every American had a chance to vote for their preferred candidate.
In the end, Ms Clinton received more than 2,800 pledged delegates and super delegates, while Mr Sanders received close to 1,900.
She has not included his name on a list of possible vice presidents, and left-leaning Massachusetts senator Elizabeth Warren is speculated to be the favourite - the pair would be the first two-woman ticket in either party’s history.
The democratic candidates are united in their condemnation of Mr Trump. Ms Clinton, Mr Sanders and Ms Warren have mocked his qualifications to be president and called him a “money grubber”.
Standing in front of his abandoned Trump Plaza casino in Atlantic City this week, Ms Clinton told the crowds that Mr Trump had vowed to do to the country what he had done to his businesses.
She speculated this would mean running the country into more national debt and then defaulting on the loan.
“It’s not about what he can build, it’s about how much he can take,” she said. “He went on twitter and said he made a lot of money in Atlantic City and left. Well, he got rich and he got out, and he thinks that’s something to be proud of.”
Independent News Service