Early voting is underway in Texas and Ohio, in what could be the final battle to determine the Democrats' presidential nominee.
Sen. Hillary Clinton is pinning the future of her struggling campaign on wins in those states on Tuesday.
She held comfortable leads in the polls as recently as two weeks ago, but her support is slipping.
The latest numbers show Sen. Barack Obama grabbing a slim lead in Texas for the first time.
Obama is also cutting into Clinton's support in Ohio, although he still trails by about seven points.
Ohio officials estimate more than people 700,000 people will cast their ballots before Tuesday's primary.