Scenario number two: Obama wins North Carolina and Indiana, which would almost certainly mean lights out for the Clinton campaign.

Clinton can keep fighting but the pressure will probably be insurmountable.

Several of her advisers have said they would counsel her to quit the race if she lost both.

However, recent surveys show Clinton is gaining momentum in Indiana and she led Obama in pre-election polls.

The third likely outcome is that candidates split the votes, with Clinton winning Indiana and Obama winning North Carolina, which would almost surely mean the struggle will continue.

This is the most likely outcome, given the demographic patterns in the Democratic race.

Obama has consistently claimed the allegiance of young voters, African Americans and voters who are better educated and wealthier.

Clinton has equally strong backing from women, less-educated voters and the economically hard-pressed.

In states where Obama's demographic groups hold sway, he wins. Where Clinton's do, she is victorious.

If the pattern holds, that makes Obama the favorite to win North Carolina and gives Clinton the edge in Indiana.

That scenario would give each candidate incentive to keep running at least until June 3, the last day of the primary season.

"As long as he wins where he's supposed to win, and she wins where she's supposed to win, the nature of the race is fundamentally unchanged," said Ben Tulchin, a Democratic pollster watching the race from the sidelines. "And so it continues."