Barack Obama and Hillary Clinton face crucial tests in their gruelling White House fight on Tuesday, when voters in Indiana and North Carolina cast ballots in the latest Democratic showdowns.

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, fire station on Monday night, urging voters to think through their decisions.

Polls close in Indiana at 7 p.m. EDT (12:00 a.m. British time Wednesday) and in North Carolina at 7:30 p.m. EDT, with results expected soon afterward.

Clinton has cut Obama's advantage in North Carolina to single digits in most polls over the past few weeks. The two run closer in Indiana, where most polls show Clinton with a slight edge.

"Obviously we hope to do as well as we can, but, you know, we started out pretty far behind," she told reporters on her campaign plane late on Monday. "I never feel confidant, I just try to do the best I can."

Obama has an almost unassailable lead in pledged delegates who will help select the Democratic nominee to face Republican John McCain in November's presidential election.

If Obama wins both contests on Tuesday, it would end Clinton's slender hopes of overtaking him in either delegates or popular votes won in the state-by-state battle and spark a fresh flood of calls for Clinton to step aside.

Clinton victories in both states could fuel doubts about Obama's electability and persuade some superdelegates - party insiders who are free to back any candidate at the nominating convention - to move toward her.

Neither candidate can win enough delegates to clinch the race before voting ends on June 3, leaving the decision to the nearly 800 superdelegates.

A split decision leaves the race largely unchanged heading to the last six contests, which have 217 delegates at stake.

OBAMA'S ROUGH STRETCH

Obama has struggled through a rough campaign stretch after last month's loss to Clinton in Pennsylvania, dogged by a furor over his comments on "bitter" small-town residents and a controversy over his former pastor, the Rev. Jeremiah Wright.

Obama, who would be the first black U.S. president, said on Monday his recent troubles "basically exhaust my problems" and he had moved past them.

Obama has won nine of 10 black votes in other states and is expected to benefit from a strong turnout in North Carolina, where blacks could make up more than one-third of primary voters.

The two Democrats, courting working- and middle-class voters suffering from an ailing economy and high gas prices, battled for the past few days over Clinton's proposal to lift the federal gasoline tax for the summer.

Obama and many economists called the plan a political gimmick, but Clinton launched an advertisement in both states on Monday questioning her rival's stance.

"What has happened to Barack Obama?" an announcer asks. "He is attacking Hillary's plan to give you a break on gas prices because he doesn't have one."

Clinton says a summer-long suspension of the tax would help Americans struggling with record gas prices in a faltering economy. Congressional leaders say there is little chance Congress will take up any gas tax proposal this year.

"Realistically, it's tough. I know that," she said late on Monday. "Do I think we can get it done, past a veto by President (George W.) Bush as the ultimate blocker?" she said. "It's obviously a very difficult challenge. But that doesn't mean you don't try."

Obama, an Illinois senator, released his own advertisement that said Clinton offered "more of the same old negative politics." He told supporters the gas tax holiday was a dishonest approach to a real problem.

"The majority of people do find me trustworthy, more than they do the other candidate," he said. "We can't solve problems if people don't think their leaders are telling them the truth."