BAGHDAD, Dec. 12 — A truck loaded with bags of wheat drove up to a crowd of poor Shiites early Tuesday, lured them close with a promise of work and exploded as they gathered around. Seventy were killed and 236 were wounded, officials said.

The attack, in a square in central Baghdad, together with corpses found by Iraqi authorities, pushed the day’s death toll across Iraq to at least 131, the highest total since a bombing killed more than 200 here last month. Shiite political leaders often point to such attacks, arguing that they, not the American military, should control security here.

The attack comes at a sensitive time for the Bush administration. The White House is weighing proposals for major changes in its policy on the war. Months of escalating violence have drained support for the occupation in the United States and left officials here and in Washington searching for a way out of an increasingly complicated war.

The Iraqi government, meanwhile, is pushing the American military to cede control of security in the capital, arguing that Iraqis knows best how to protect themselves. American commanders, however, fear the Shiite-dominated government will use the predominantly Shiite army as a weapon against Iraq’s embattled Sunni Arab minority.