Hoosiers and Tar Heels have a chance to reshape the presidential race today in balloting that will award the largest collective cache of delegates and popular votes remaining on the long Democratic nomination calendar.

Just hours before the polls were to open, Sens. Hillary Rodham Clinton of New York and Barack Obama of Illinois shuttled between Indiana and North Carolina, continuing their arguments over gasoline taxes.

In commercials and in person, Mrs. Clinton defended her call for a summer fuel tax holiday in the face of her rival's characterization of the plan as a political gimmick. Oil futures reached a record of more than $120 a barrel yesterday, raising concerns about even higher prices for gasoline.

While the two states could produce results that would dramatically alter their competition, they are not likely to end it. Both sides predicted that the protracted race would continue at least through the final primaries in June. That means that the political focus will shift tomorrow from the two states with puzzling nicknames to West Virginia. The Mountaineer State is expected to produce good news for Mrs. Clinton a week from now. Whether that will amount to added momentum or merely a forgotten consolation prize depends on today's results.

Surveys of the states voting today suggested that Mrs. Clinton had the advantage in Indiana, the Hoosier State, while Mr. Obama held a clear but diminished edge in North Carolina. A split decision would send the competition lurching toward its final rounds in a handful of relatively smaller states. Should Mr. Obama manage to win both, despite weeks of the most debilitating developments of his campaign, it would provide a damaging rebuttal to the electability argument that Mrs. Clinton has pressed with increasing strength through her big victories in Ohio and Pennsylvania.

Even a Clinton sweep would not be enough to put her on the path to overcoming Mr. Obama's lead in elected delegates. It would, however, add to the momentum of her big-state wins, bolstering both her fundraising and her argument for her underdog campaign to the superdelegates likely to be the final arbiters of the Democratic nomination.

According to The Associated Press tally, Mr. Obama had 1,745.5 total delegates to Mrs. Clinton's 1,608 before today's contests. But as the voting neared, Mr. Obama continued to close Mrs. Clinton's lead among superdelegates with two more pledges of support yesterday, leaving him trailing, 269-255. Under current Democratic rules, it would take 2,025 votes to secure a convention majority, but that benchmark could change if Michigan and Florida's currently banned delegations are seated.

The superdelegates remain the key to Mr. Clinton's hopes, although the tide in that body of party elders has been running against her for months. According to a separate informal count by MSNBC, since Mrs. Clinton's win in the Pennsylvania primary, Mr. Obama has picked up 21 superdelegates to her 11.

While both of today's contests are expected to be close, Mrs. Clinton will move on to West Virginia, where she is expected to campaign later in the week, as the overwhelming favorite. A new Rasmussen survey released yesterday showed her leading, 56 percent to 27 percent, just over a week before a primary that, like today's contests, allows voting by independents.

Below the national political radar, Mountaineer State voters in fact have been casting presidential ballots since April 23 in an early voting period that ends Saturday.

"I would say, in general, people are excited,'' said Cheryl A. Brown, associate professor of political science at Marshall University. "It's the first time in years that the West Virginia primary has had the potential to make a difference on the nominee."

Dr. Brown noted that the last nationally significant presidential primary in the state was in 1960 when John Kennedy scored a come-from-behind victory against Hubert Humphrey. In that race, Kennedy faced stiff resistance over his Roman Catholic religion but prevailed by casting the race as a test of tolerance versus intolerance.

"I refuse to believe that I was denied the right to be president the day that I was baptized,'' he told a West Virginia audience.

His campaign also spent a lot of money. In a self-mocking speech in New York's annual Al Smith dinner, the future president referred to West Virginia campaign as he quoted a telegram supposedly sent by his father, the millionaire, Joseph P. Kennedy.

"Don't buy a single vote more than necessary,'' he deadpanned. "I'll be damned if I'm going to pay for a landslide."

While acknowledging its underdog status next week, the well-funded Obama campaign is not conceding West Virginia. Both candidates have ads on the state's television this week. Following the pattern of earlier states, however, Mr. Obama was on television earlier and with a significantly heavier presence than was Mrs. Clinton. His logistical advantage extends to grass-roots organizing. He has opened with nine offices throughout, twice that of the Clinton campaign.

Mr. Obama also has the support of Sen. Jay Rockefeller and U.S. Rep. Nick Rahall. Gov. Joe Manchin and the state's senior Democrat, Sen. Robert Byrd, are neutral in the race so far.

Both of the Democratic candidates have made appearances in the state as have former President Bill Clinton and the Clintons' daughter, Chelsea. While West Virginia's is an open primary, the state has also seen the kind of Democratic registration gains that have occurred regularly in earlier primary states.

Aides to both said that their prime focus for the balance of the week would be on the early voting.