WASHINGTON (AP) — Barely four months before the nation votes, one of the biggest factors in the fight for the White House still is a mystery. That will change on Thursday.

The Supreme Court's expected ruling on President Barack Obama's sweeping federal health care law will shape the contours of the presidential campaign through the summer and fall. Both Obama and Republican rival Mitt Romney are primed to use the ruling — whatever it is — for political gain.

Obama expresses confidence the court will uphold his signature legislative initiative. But he won't be shocked if a conservative majority overturns the most controversial provision, those familiar with his thinking say. Romney aides say the Republican candidate will get a political boost if the court strikes down the measure. But they don't want celebrations that could alienate voters who could lose health care benefits through the decision.