Their concerns that the legislation would harm the nation's most vulnerable and cause many Americans to become uninsured have thrust into stark relief the ideological fault lines within the GOP. Though Senate conservatives were the first to threaten to torpedo the bill, contending that it is too generous, the potential loss of nearly half a dozen moderate lawmakers' votes may be the main hurdle. Since the bill will get no support from Democrats, Senate Majority Leader Mitch McConnell can afford defections from no more than two Republicans as he tries to bring it to a vote this week.