Baghdad ? An al-Qaida front group threatened to assassinate Sunni leaders who support American troops in Iraq as a Shiite bloc loyal to radical cleric Muqtada al-Sadr defected Saturday from the Iraqi government’s parliament base.

The two developments cast doubt over prospects for political and military progress in Iraq as the U.S. Senate gears up for a debate next week on Democratic demands for deeper and faster troop cuts than President Bush plans.

The threat against Sunni leaders came from the Islamic State of Iraq, which claimed responsibility for the assassination Thursday of Abdul-Sattar Abu Risha, the mastermind of the Sunni Arab revolt against al-Qaida in Anbar province. Bush met Abu Risha at a U.S. base in Anbar this month and praised his courage.

Also Saturday, car bomb exploded in a mostly Shiite area of southwest Baghdad, killing at least 11 people lined up to buy bread at a bakery. Two of the dead were children, police said.