Crucial contests take place in delegate-rich Ohio and Texas, and smaller primaries in Vermont and Rhode Island. John McCain seals the Republican nomination with wins in all four states. His only remaining rival - Mike Huckabee - bows out of the race. In the Democratic contests, Hillary Clinton revives her campaign with triumphs in Ohio, Texas and Rhode Island - her first victories since Super Tuesday. Barack Obama wins in Vermont.

"Stand up with me, my friends, stand up and fight for America - for her strength, her ideals, and her future. The contest begins tonight."
John McCain claims the Republican nomination

We're going on, we're going strong and we're going all the way Hillary Clinton At-a-glance: 3 March

"We started this effort with very little recognition and virtually no resources. We ended with slightly more recognition and very few resources."
Mike Huckabee concedes

"The people of Ohio have said it loudly and clearly. We're going on, we're going strong and we're going all the way."
Hillary Clinton celebrates her Ohio victory.

"No matter what happens tonight, we have nearly the same delegate lead as we did this morning, and we are on our way to winning this nomination."
Barack Obama

A number of state polls in Ohio and Texas have been published in the last few days, and Tuesday's results will be the definitive test of how accurately they reflected the final tallies.

Seven polls conducted in the first days of March gave Hillary Clinton moderate leads over Mr Obama in Ohio. At one extreme the American Research Group (ARG) gave her a 14-point lead, 56-42, but in an eighth poll, from Zogby, Mr Obama had a 47-45 lead over Mrs Clinton.

In Texas, the Democratic contest was much closer, polling suggested.

Mrs Clinton led in the latest polls from ARG (50-47), InsiderAdvantage (49-44), PPP (50-44) and Public Strategies (46-45).

But Mr Obama had the edge in polls by Rasmussen (48-47), Survey USA (49-48) and Zogby (47-44)

All surveys of Republican voters in the two states correctly suggested that John McCain was heading for a comfortable victory.

John McCain celebrates in front of supporters. A banner indicates the number of delegates he needed to win to clinch the nomination