Barack Obama and Hillary Rodham Clinton fought for a breakout in their eyeball-to-eyeball Democratic duel while Republican John McCain hoped to bury his rival’s presidential hopes in a blur of voting Tuesday from Alaska to the Atlantic. An enormous cache of delegates was at stake — not enough to clinch a nomination but plenty enough to mint a runaway favorite, or even twoSen. John McCain seeks to seal the GOP presidential nomination in across-the-nation voting Tuesday.