Polls are open in all four of the U.S. states holding primary elections today Tuesday as U.S. Senators Hillary Clinton and Barack Obama battle for support for the Democratic Party nomination.

Texas and Ohio are the two key states in Tuesday's balloting, because they have many more delegates than the small northeastern states of Vermont and Rhode Island. Delegates choose the party's nominee for the presidential race.

Clinton lags behind Obama, with 11 straight losses in primaries and caucuses. Former U.S. President Bill Clinton has said his wife needs to win both Texas and Ohio to keep her campaign alive.

On the Republican side, Senator John McCain seems on the verge of clinching his party's nomination.

Senator McCain could secure the Republican Party nomination with victories Tuesday. He has a commanding lead over former Arkansas Governor Mike Huckabee.