President Trump has never been known to back away from a political fight, and choosing to nominate a replacement for Justice Ruth Bader Ginsburg immediately was in character. Now he probably ends his term with the most consequential fight yet. The result could be an enduring 6-3 conservative majority on the Supreme Court or an embarrassing political defeat just as voters head to the polls to decide if he merits a second term. It could also trigger a successful Democratic court-packing effort, ushering in a new era of liberal dominance as early as next year.That last possibility is a sign both of the escalating "legitimacy war" between the two major parties and of the stakes of the final battle of Trump's term.His floor general is Senate Majority Leader Mitch McConnell. The Kentucky Republican is a canny legislative tactician who helped the president beat impeachment and prevail in two previous Supreme Court confirmation battles. Together, they have reshaped the federal judiciary along more conservative lines, nominating and confirming more than 200 judges. McConnell vowed Trump’s nominee would get a Senate vote the night Ginsburg died, before the president himself was willing to go beyond expressing condolences. Arrayed against them will be the full force of the Democratic Party and its media allies, which will mobilize against Ginsburg’s seat being moved over to the conservative bloc, and all the liberal interest groups they can enlist. “Donald Trump...