WASHINGTON — After seven years of emphatic campaign promises, Senate Republicans demonstrated Wednesday they don’t have the stomach to repeal “Obamacare” when it really counts, as the Senate voted 55-45 to reject legislation undoing major portions of Barack Obama’s law without replacing it. Seven Republicans joined all Democrats in rejecting an amendment by Rand Paul of Kentucky that would have repealed most of former President Obama’s health care law, with a two-year delay but no replacement.