Musharraf received a setback when more than two dozen of his parliamentary allies ditched him and said they would support the ruling alliance on the impeachment issue.
In the Senate, the pro-Musharraf Pakistan Muslim League holds 38 of the 100 seats, on paper enough to block impeachment. But party sources admit that some 18 of its senators have refused to give support to Musharraf.
All four of Pakistan's provincial assemblies have passed resolutions calling on Musharraf to either resign or face impeachment.
In recent months, Washington has actively engaged with General Kayani, who took command of the Pakistani Army last November. High-level U.S. military and intelligence officials have gone to Islamabad to promise military aid and to press for help fighting al Qaeda and Taliban extremists along the border between Afghanistan and Pakistan.
There have been concerns in Washington about the political stability of Pakistan. Even as the country faces the Islamic insurgency in its tribal areas, Musharraf's ouster could widen rifts among coalition partners in the current government, with jockeying for office by Sharif and Bhutto's widower, Asif Ali Zadari, himself controversial because of past, disputed corruption charges.