Barack Obama sought a knockout against rival Hillary Rodham Clinton as voting began yesterday in crucial races in Texas and Ohio, where the former first lady desperately needs a win to salvage her once-powerful candidacy for the Democratic presidential nomination.

Voters in two other states - Vermont and Rhode Island - also cast ballots in contests that offer a total of 370 delegates. But Texas and Ohio are pivotal, providing Clinton with what could be her last chance to end Obama's string of 11 wins and keep alive her hope to become the US's first female president.

Republicans also compete in the same four primaries, and a strong showing by presumptive nominee John McCain could allow him to clinch the nomination that is already well within his grasp.