Advocacy groups spending millions of dollars to influence the 2012 election now face the prospect of having to reveal their secret donors, after a federal appellate court panel refused to block a lower-court order requiring the disclosure. In a 2-to-1 decision issued Monday evening, a U.S. Court of Appeals panel here declined to stay a ruling by a federal judge requiring tax-exempt organizations that run election-related television ads to disclose their donors. The panel’s decision was a significant victory for campaign finance reform advocates who have been fighting against the deluge of money — much of it from undisclosed donors — that has flooded the political landscape in the wake of several Supreme Court decisions, including the 2010 Citizens United case.