WASHINGTON — Sharply divided along familiar lines, the Supreme Court took up a politically charged new challenge to President Barack Obama's health overhaul Wednesday in a dispute over the tax subsidies that make insurance affordable for millions of Americans. The outcome in what Justice Elena Kagan called "this never-ending saga" of Republican-led efforts to kill the Affordable Care Act appears to hinge on the votes of Chief Justice John Roberts, whose vote saved the law three years ago, and Justice Anthony Kennedy. Roberts said almost nothing in Wednesday's 85 minutes of lively back-and-forth, and Kennedy, who voted to strike down the health law in 2012, asked questions of both sides that made it hard to tell where he might come out this time.