"She got major applause when she said would not name anybody to the Supreme Court unless she has assurances that they would overturn" the decision, said one attendee, who, like others, requested anonymity to describe the private session.
If the make-up of the court does not change by 2017, four of the justices will be 78 years of age or older by the time the next president is inaugurated.
Clinton’s pledge to use opposition to Citizens United as a litmus test for Supreme Court nominees echoes the stance taken by Sen. Bernie Sanders of Vermont, who is challenging her for the Democratic nomination.
“If elected president, I will have a litmus test in terms of my nominee to be a Supreme Court justice,” Sanders said on CBS’ “Face the Nation” on Sunday. “And that nominee will say that we are all going to overturn this disastrous Supreme Court decision on Citizens United because that decision is undermining American democracy. I do not believe that billionaires should be able to buy politicians.”
On Thursday, Clinton also reiterated her support for a constitutional amendment that would overturn Citizens United, a long-shot effort that is nonetheless popular among Democratic activists.
"She said she is going to do everything she can," the attendee said. "She was very firm about this – that this Supreme Court decision is just a disaster."
A campaign spokesman did not immediately respond to a request for comment.
Clinton has made overhauling the current campaign finance system one of the major planks of her campaign, even as she has tacitly endorsed the efforts of two big-money super PACs working to help get her elected in 2016 -- Priorities USA Action and Correct the Record. Advisers have said that they cannot reject such vehicles when they are being vigorously embraced by the Republican field.
Still, the prominent role that wealthy donors are expected to play in boosting her bid could make it hard for Clinton to cast herself as a champion of campaign finance reform, particularly in the eyes of some advocates who remember the soft-money scandals of the Bill Clinton administration.
On Thursday, the former secretary of state spoke and took questions for about 45 minutes in a converted warehouse on a pier in Brooklyn's Red Hook neighborhood, with sweeping waterfront views of the Manhattan skyline and the Statue of Liberty. The fundraisers on hand for the meeting -- a mix of longtime Clinton backers and a small number of bundlers who played major roles in the campaigns of President Obama -- had all raised at least $27,000 for her campaign.
In response to questions from the group, Clinton spoke about the need for renewable energy and strongly endorsed Obama’s free community college plan.
She avoided taking a position on the controversial Trans-Pacific Partnership trade accord, saying she first wants to see what comes out of Congress.
Afterward, Clinton stayed and shook hands for 45 minutes. There was no sense that old Clinton loyalists had higher ranking than newer supporters, attendees said. “She spent time with every single person -- new friends and old friends equally," said a second participant.
Robert Barnes and Karen Tumulty contributed to this report.