Senate Democrats are unlikely to let GOP leaders fill the seats vacated by Sen. Arlen Specter (Pa.) on the Judiciary and Appropriations panels with another Republican, deciding instead to keep the overall size of the committees the same for the remainder of the 111th Congress.

The move means Democrats will see their margins grow by one on both panels, while Republicans will see their numbers shrink by one. Specter bolted from the GOP last week to join the Democratic Party. Republicans had hoped to fill Specter’s vacancies on Judiciary and Appropriations with a new GOP Senator, but Democrats thus far are refusing.

Senate Majority Leader Harry Reid (D-Nev.) and Minority Leader Mitch McConnell (R-Ky.) held a series of discussions on the subject beginning Thursday.

According to sources familiar with the talks, Democrats have been unwilling to adjust the makeup of just two committees. Aides said Democrats have argued that either a Republican must be dropped from two other committees to allow new seats on Judiciary and Appropriations, or that the Senate adopt an entirely new organizing resolution for the session. An organizing resolution, adopted at the beginning of a new Congress, sets the ratios on all panels.

Currently, that resolution is based on a ratio of 58 Democrats to 42 Republicans. If it were revised, Democrats would almost certainly demand it be done to reflect a 60 -40 split, which would mean cuts to Republicans on committees across the board.

Prior to Specter’s switch, Democrats had 17 Appropriations Committee members to the GOP’s 13. The Judiciary panel, meanwhile, had an advantage for Democrats, 11 to 8. With Specter on board, the Democrats will have 18 seats on Appropriations and 12 on Judiciary.