Last year several pro-Republican advocacy groups degraded the Congressional elections by spending at least $138 million in secret donations on advertisements. The public did not know which lobbying interests gave money, or how much, or what they would demand in return. But the donations became a significant factor in the Republican gains in the House and the Senate.

Now several prominent Democrats are abandoning the high ground and have decided to raise millions of their own secret dollars. They have promised they will again try to pass a law preventing this secrecy if they win. (They were stymied in an earlier attempt by a Republican Senate filibuster.) Whatever they gain in money, they stand to lose far more by giving up principles that President Obama and party leaders once claimed to cherish.

Bill Burton, who until February was Mr. Obama’s deputy press secretary, said last week that he would help lead a group called Priorities USA, which will raise unlimited money from undisclosed sources to aid in the president’s re-election campaign. The initial money will come from the Service Employees International Union and Jeffrey Katzenberg, the Hollywood producer, but more will inevitably begin to flow in from other unions and wealthy Democrats.

Mr. Obama has long claimed to champion transparency and denounced the secret-money sluice operated by Republicans last year as a “threat to democracy.” As he said in October, “The American people deserve to know who’s trying to sway their elections, and you can’t stand by and let the special interests drown out the voices of the American people.” Last year, speaking for the administration, Mr. Burton called for a “bright light” to shine on the shadowy groups.