Two more Democrats have broken with President Obama on his plan to institute an executive order that would force contracting companies seeking federal contracts to reveal any political contributions made by the firm’s executives. Not only would the order likely have a chilling effect on protected political speech, but it could also create an atmosphere in which federal contracts are rewarded based on the politics of the contracting firm, as opposed to the firm’s merits.

Sen. Joe Lieberman, an independent who caucuses with the Democrats, and Sen. Claire McCaskill are the two latest Democratic defectors. They joined Republicans Sen. Rob Portman and Sen. Susan Collins in signing a letter to President Obama objecting to the plan.

Sen. Portman, who spearheaded the letter, asked the president to “abandon this ill-conceived Executive Order immediately.”

“Politics has no place in the award of federal contracts, and this order risks suppressing disfavored political speech,” he said in a press statement today.

Another prominent Democrat, House Minority Whip Steny Hoyer, has also spoken out strongly against Obama’s proposal. “I’m not in agreement with the administration on that issue. The issue on contracting ought to be on the merits of the contractors’ application and bid and capabilities,” Hoyer said.

The public criticism from fellow Democrats will no doubt be a headache for Obama. But the fact that this opposition is coming from the two Democrats in the Senate with jurisdiction on this issue will make it much more difficult for the president to go ahead with his plan. Lieberman is the chair of the Homeland Security committee, and McCaskill the chair of its contracting subcommittee, which gives them leverage on this issue.