Democratic Sens. Dianne Feinstein (Calif.) and Frank Lautenberg (N.J.) on Tuesday will support a motion to begin debate on a controversial campaign finance reform bill, even though both lawmakers oppose the measure and are likely to vote against it on final passage barring significant changes.

Democratic aides said Lautenberg will vote for cloture this afternoon, while Feinstein said she would “because it’s a motion to proceed.”

Senate leaders still need one more vote to begin debate on the DISCLOSE Act. Republicans are promising to filibuster the measure. The Democratic vote count is complicated further by the absence of Sen. Joe Lieberman (ID-Conn.), who is away from Washington, D.C., dealing with a death in the family.

Democratic Conference Vice Chairman Charles Schumer (N.Y.) is the chief architect of the DISCLOSE Act, which is aimed at partially reversing the Citizens United Supreme Court decision on campaign finance spending. All Republican Senators oppose the measure; Feinstein and Lautenberg have opposed it because of a carve-out approved in the House for the National Rifle Association. The Senate was scheduled to vote on the motion Tuesday afternoon.