WASHINGTON  In the great health care debate of 2009, President Obama has cast himself as a cold-eyed pragmatist, willing to compromise in exchange for votes. Now ideology  an uprising on the Democratic left  is smacking the pragmatic president in the face.

Stung by the intense White House effort to court the votes of moderate holdouts like Senator Joseph I. Lieberman, independent of Connecticut, and Senator Ben Nelson, Democrat of Nebraska, liberals are signaling that they have compromised enough. Grass-roots groups are balking, liberal commentators are becoming more critical of the president, some unions are threatening to withhold support and Howard Dean, the former Democratic Party chief, is urging the Senate to kill its health bill.

The White House scrambled Thursday to tamp down the revolt, which has been simmering for weeks but boiled over when the Senate Democratic leadership, bowing to Mr. Lieberman, scrapped language allowing people as young as 55 to buy into Medicare.

David Axelrod, Mr. Obama’s senior adviser, began the day by calling in to MSNBC to urge the party to hold together, warning of a “tragic outcome” if Democrats failed to pass a bill that the White House says would expand health coverage while reining in costs.