Washington: Democrats Barack Obama and Hillary Clinton face crucial tests in their White House fight on Tuesday, when voters in Indiana and North Carolina cast their ballots.

The two states, with a combined 187 delegates to the August nominating convention at stake, are the biggest prizes remaining in the tight Democratic nominating race.

After Tuesday, only six contests will be left.

"The stakes are high and the consequences are huge," Clinton told supporters at a New Albany, Indiana on Monday night, urging voters to think through their decision.

If Obama wins both contests on Tuesday, it would end Clinton's slender hopes of overtaking him.

Clinton victories in both states could fuel doubts about Obama's electability.