BAGHDAD, Iraq, March 22 — Iraq's most influential Shiite cleric, Grand Ayatollah Ali al-Sistani, has warned of "dangerous consequences" if the United Nations endorses the American-sponsored interim constitution for an independent Iraq that was adopted over Shiite protests two weeks ago. The warning came in a letter released by Ayatollah Sistani's office on Monday, four days after it was delivered in New York to Lakhtar Brahimi, the chief United Nations envoy to Iraq. It amounted to a warning that the ayatollah's followers, by far the most powerful political bloc in Iraq, could move to paralyze American plans for a smooth...