The only way the GOP can really deal a mortal blow is by taking back the White House in 2013, these Republicans say — but, even then, the clock would be running out, with the law well into its fourth year of implementation and just a year away from when major benefits kick in. [...] it’s a multipronged attack aimed at constantly putting President Barack Obama and congressional Democrats in the uncomfortable position of defending a law that has yet to win broad public support. An amendment last week by Sen.