Mitt Romney isn't singing anymore. A little more than two weeks after his off-key but enthusiastic renditions of "America the Beautiful" captured the spirit of a candidate who had won the Florida primary and seemed on the verge of locking down the Republican nomination for president, Romney is back in lackluster mode. It's not just that Romney is facing a surprising challenge in his native state of Michigan from Rick Santorum, the former Pennsylvania senator who vaulted into contention in the state-by-state nomination battle with wins in Missouri, Minnesota and Colorado. What alarms many Republican Party leaders and strategists are...