Lawmakers loyal to radical Shiite cleric Muqtada al-Sadr now say that they are pulling out of the coalition forged by Iraqi Prime Minister Nouri al-Maliki.
This leaves the prime minister's government short of a parliamentary majority and presents a new challenge for al-Maliki.
In addition, an al-Qaida front group threatened to assassinate Sunni leaders who support American troops in Iraq.
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.
From NPR and The Associated Press reports