Senator John McCain has retreated from the campaign finance fight, once his signature issue. He may finally be ready to get back in. He told the newspaper The Hill that he had been talking with Senator Sheldon Whitehouse, Democrat of Rhode Island, the sponsor of a much-needed bill requiring timely public disclosure of donors writing checks of $10,000 and more for election ads, plus “stand-by-your-ad” identifications from the five biggest donors. [...] The public has a clear and urgent need to know who are the hidden donors bankrolling the salvos of attack ads. But the Senate measure is reported stalled, with no Republican willing to join as a co-sponsor. [...] Mr. McCain could help drive the issue by defying his party’s monolithic dedication to unlimited and undisclosed political donations.