JOHNSTOWN, Pa., Feb. 23 — As Democratic Congressional leaders escalate their challenge to the Bush administration’s policy in Iraq, one of the most influential players will be that unlikely darling of the antiwar left, Representative John P. Murtha of Pennsylvania. Mr. Murtha’s power comes not just from his role as chairman of the Defense Appropriations Subcommittee, in charge of the huge spending bill for the war that will advance in the House in March. It also derives from his close alliance with Speaker Nancy Pelosi, and from his popularity, ever since he called for an end to the war in...