Barack Obama approaches Tuesday's voting in four states as a chance to drive rival Hillary Rodham Clinton out of the Democratic presidential contest for good.
Clinton looks at the same contests as a way to end Obama's streak of 11 straight victories and keep her candidacy going.
Both camps acknowledge that split decisions and close votes could prolong the battle for at least another month -- or even longer.
Polls open in a few hours in Texas and are already open in Rhode Island, Vermont and Ohio.
Ohio voters are facing bad weather -- freezing rain and the threat of flooding.
Republican front-runner John McCain could officially clinch the GOP nomination with a good showing. Mike Huckabee says he won't drop out of the race until someone has 1,191 confirmed delegates.