WASHINGTON (Reuters) - A third Republican U.S. senator on Tuesday said she could not back any legislation to repeal Obamacare without a replacement plan, effectively blocking Republican leaders’ efforts to bring up a narrower repeal-only bill after earlier legislation collapsed. “To repeal there has to be a replacement. There’s enough chaos already, and this would just contribute to it,” Senator Lisa Murkowski of Alaska told reporters, saying she would vote no on any effort to take up legislation to repeal the Affordable Care Act enacted in 2010 under former President Barack Obama. 