U.S. Democratic Party presidential hopefuls Hillary Clinton and Barack Obama are battling for support in four presidential nominating contests.
Texas and Ohio are the key states in Tuesday's races, offering far more delegates than the small northeastern states of Vermont and Rhode Island. Record turnout is expected.
For the Republicans, Senator John McCain is on the verge of clinching his party's nomination, as the veteran Arizona lawmaker maintains a commanding lead over former Arkansas Governor Mike Huckabee.
Clinton, slightly behind Obama in the number of delegates needed to secure the Democratic Party nomination, hopes to halt Obama's winning streak after his 11 straight victories in nominating contests.
Polls indicate a tight race between Obama and Clinton in Texas and Ohio. Surveys show Obama leading Clinton in Vermont, while Clinton is ahead in Rhode Island.
Clinton indicated in a television appearance late Monday (on Comedy Central) that she may remain in the race even if she does not win Texas and Ohio.