The Supreme Court ruling on President Barack Obama's health care overhaul, now just hours away, marks a key pivot point in the presidential race. But neither side knows which direction the high court's decision will turn the contest.

Anticipation of Thursday's decision could be unnerving for the president and his Republican challenger, Mitt Romney, whose opposition to the law has become a central pillar of his campaign.

The court could uphold the health care law, strike it down or deem the requirement that most Americans carry health insurance unconstitutional while retaining other aspects of the law.

The announcement is expected to be followed by a barrage of advertisements and fundraising appeals from Democrats and Republicans, all trying to cast the decision in the most advantageous light for their candidates.