Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE called on Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s supporters to consider voting for Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE solely over her position on campaign finance reform.

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While campaigning for Clinton in Fort Collins, Colo., on Monday, Sanders made a pitch to the GOP nominee's supporters as the Vermont senator bemoaned the 2010 Supreme Court ruling of Citizens United and reiterated his call to get “big money out of politics.”

Sanders touted Clinton’s promise to propose a constitutional amendment in her first 100 days if elected president to overturn that ruling. He also noted that Clinton will nominate a Supreme Court justice to fill the vacancy who will support overturning Citizens United and that Trump will nominate a “conservative” who will likely help it “remain intact.”

“This issue of Citizens United is a profound issue impacting the entire country,” Sanders said. “I say to Mr. Trump’s supporters: most of you understand that system is rigged in favor of wealthy and power.”

“Think about voting for Hillary Clinton on this issue alone,” he continued.

Campaign finance reform was central to Sanders’s presidential campaign in the Democratic primary. Since ending his bid, the Vermont Independent has stumped for Clinton across the country and is stepping up his appearances in the final weeks of the campaign.

Trump has sought to court Sanders’s loyal supporters, saying that the two share common ground on opposition to international trade deals. The GOP nominee has attacked Clinton for once supporting the Trans-Pacific Partnership (TPP) and calling it the “gold standard,” although she has voiced opposition to the President Obama’s signature trade deal in its current form.

Clinton goes into the final three weeks before Election Day with a comfortable lead over Trump in the wake of sexual assault allegations against the real estate mogul. She holds a more than 6-point edge in a RealClearPolitics average of national polls, with some showing her ahead by double digits.