Two former chairmen of the U.S. Securities and Exchange Commission (SEC) urged the regulator to force companies to disclose campaign donations, pressuring the agency to weigh in on a politically explosive issue ahead of the 2016 presidential election.

In a bipartisan letter to SEC Chair Mary Jo White on Wednesday, Arthur Levitt and William Donaldson said corporate political spending is of "paramount public interest and growing concern to investors." They pointed to a record 1.2 million comment letters supporting a 2011 petition by a group of securities law professors as evidence that the agency should act.

"The Commission's inaction is inexplicable," Levitt and Donaldson said in the letter, which was also signed by former SEC commissioner Bevis Longstreth. "It flies in the face of the primary mission of the commission, which has since 1934 been the protection of investors."