Tuesday's primary unlikely to end Democratic race

He missed a chance to close out the Democratic presidential nominating fight by losing Ohio to rival Hillary Rodham Clinton, then lost again in Pennsylvania. His big lead over Clinton in national polls disappeared.

Worse, the Rev. Jeremiah A. Wright Jr. -- Obama's longtime pastor -- resurfaced, spitting fire during a National Press Club appearance. The Illinois senator was badly burned in the process.

Yet for all that, Obama remains a strong favorite to win the nomination. The simple reason is mathematics: His lead among elected delegates makes it virtually impossible for Clinton to win without having the nomination handed to her by party insiders, the so-called superdelegates.

"We have demographic gridlock, and we have rules that make it very difficult to catch up when you're behind," said Bill Carrick, a neutral Democratic strategist, referring to the candidates' distinct appeal to separate constituencies and the proportional way Democrats award their delegates. "It all conspires to make a very rigid, inflexible process."

Which does not mean the contest is likely to end soon.

The next test comes Tuesday, when Indiana and North Carolina hold their primaries. Barring the unexpected -- a blowout in either state, or twin victories by either Obama or Clinton -- the probable outcome is a continued stalemate.

That would give each candidate incentive to keep running at least until June 3, the last day of the primary season: Obama because of his seemingly insurmountable lead in nominating delegates and the popular vote, and Clinton because of doubts sown in recent weeks about Obama's general-election viability.

"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."

But as the election season grinds toward a close, the pressure on Clinton to change the dynamic of the contest has grown more pronounced. Nearly 90% of the pledged delegates have been chosen. Even if she wins all the remaining primaries in a landslide, the New York senator would still need to corral an overwhelming proportion of the party's undeclared superdelegates -- the party leaders and others with an automatic vote at the Democratic convention -- to seize the nomination.