Washington ? Sens. Hillary Clinton of New York and Barack Obama of Illinois are running roughly even nationally as the battle for the Democratic nomination heads into Tuesday’s big round of primaries and caucuses, while Sen. John McCain of Arizona has jumped to a dominating lead over his remaining rivals in the Republican race, according to a new Washington Post-ABC News poll.

Two days before voters in 24 states go to their polling places, 47 percent of likely Democratic voters said they back Clinton and 43 percent said they support Obama, with neither candidate decisively benefiting from the departure of former Sen. John Edwards of North Carolina from the race. By contrast, McCain’s wins in primaries in South Carolina and Florida and the winnowing of the Republican field have had a dramatic result: The senator from Arizona is now the clear front-runner for his party’s nomination.

McCain leads former Massachusetts Gov. Mitt Romney 48 percent to 24 percent among probable GOP voters as he continues to rapidly consolidate support, particularly among moderates and liberals. Former Arkansas Gov. Mike Huckabee runs third in the new poll with 16 percent, and Rep. Ron Paul of Texas is fourth at 7 percent.

The Democratic and Republican hopefuls have been furiously crisscrossing the country seeking out votes in advance of Super Tuesday. More primaries and caucuses are on Feb. 5 than on any previous single day in a nominating contest; about half the delegates needed to secure each party’s nomination are at stake.

McCain’s big lead in this new national poll matches a wave of increasing support seen in state polls, which, coupled with the GOP’s winner-take-all rules, gives him the opportunity to effectively wrap up the nomination with a strong showing Tuesday.

The basic fault line between Clinton and Obama remains leadership and experience versus a new direction and new ideas.

While Clinton has the edge on the issues voters say are most important to them, and enjoys a wide lead on the question of who is a stronger leader, Obama now holds a seven-percentage-point advantage as the candidate who would do the most to bring needed change to Washington.

In hypothetical general-election matchups, both Democrats run neck and neck with McCain, and both lead Romney by double digits.