Even Connecticut Rep. Chris Shays–a longtime war supporter who voted against the resolution Friday– said he will be willing to look at a “Plan B” for troop withdrawal if the situation doesn’t change very soon. “I think you need to start to see some noticeable differences in four months,” he says. “You need to see trend lines that are going up and not down.” What trend lines? A decline in violence, signs that the Iraqi troops are protecting Sunni neighborhoods and some diplomatic overtures by the administration to Syria in particular. But the biggest Republican to watch is Sen. John McCain, perhaps the Iraq war’s most passionate advocate on Capitol Hill, who also happens to be running for president. For all his ardent backing of the troop surge, McCain in recent weeks has signaled that he won’t maintain his backing indefinitely if he concludes the United States is a mere bystander in a hopeless civil war. “I think he’s telegraphed his moves pretty clearly on this,” says one close McCain advisor who said the senator would then call for a troop withdrawal. And that, says another Senate aide, “would be the biggest bellwether of all.”