US Senator Scott Brown was elected on a promise to be the 41st vote that Republican senators needed to block Democrats from passing a major overhaul of the nation’s health care system.

But Democratic leaders devised a way to skirt a Republican filibuster, and the Affordable Care Act ­became law in spite of Brown’s 2010 victory. Almost three years later, that law remains a bright line in Massachusetts politics, separating Brown and his challenger for ­reelection, Democrat Elizabeth Warren, on health care: He would repeal it, she would keep it in place.