Investors have pushed more than 120 public companies in the U.S. to reveal previously confidential details of their political spending even as regulators remain deadlocked over whether to mandate such disclosures.

Thirty-one companies — up from 28 last year — will hold shareholder votes in 2013 on resolutions calling for disclosure of how much they spend to influence politics, according to the Washington-based Center for Political Accountability, which helps write the resolutions. The first of those votes, at Humana Inc.'s annual meeting, is scheduled for today.

Another 16 companies, including Boeing Co., KeyCorp, Qualcomm Inc., and Southwest Airlines Co., have agreed to provide investors with data on their political spending this year in an effort to avoid shareholder votes mandating the disclosures.