Senator John McCain returned from the political abyss to clinch the Republicans' White House nomination late Tuesday as Democrats Barack Obama and Hillary Clinton slugged it out in Ohio and Texas.

Capping one of the most extraordinary political comebacks of recent years, McCain defeated former Arkansas governor Mike Huckabee in all four of the states holding primary ballots - Texas, Ohio, Vermont and Rhode Island.

Screams of joy broke out among the 200-odd McCain faithful crowding the ballroom of a downtown Dallas hotel after US network projections gave him victory.

But a giant billboard bearing the magic number of 1,091 delegates - the Republican winning line - lay covered in blue cloth awaiting a ceremonial unveiling.

Huckabee, an ordained Baptist preacher who had appealed to the southern Bible Belt, said he had telephoned McCain to offer his acclaim and support.

"I extended to him not only my congratulations, but my commitment to him and to the party to do everything possible to unite our party, but more importantly to unite our country so that we can be the best nation we can be, not for ourselves, but for the future generations," he said.

The 71-year-old Arizona senator was scheduled to head to the White House on Wednesday to receive the blessing of President George W. Bush.

But on the Democratic side, the race was shrouded in mystery and controversy.

Obama drew first blood in the liberal northeastern state of Vermont to extend his month-long winning streak to 12 nominating contests.

But the former first lady hit back by taking Rhode Island, according to the projections.

That left the bigger prizes of Ohio and Texas still up for grabs, two states that former president Bill Clinton has said his wife must win to sustain her dream of following him into the Oval Office.

Bad weather and a ballot shortage forced an hour-long extension to voting in parts of Ohio.

In Texas, the day-long primary was being followed by election caucuses in a quirky two-step process that was engulfed in controversy.

Clinton's campaign accused Obama's camp of engaging in "outrageous" and "undemocratic" tactics in the Lone Star state, including locking her supporters out of caucuses.

"What is happening tonight is just truly an outrage," Clinton's Texas state director Ace Smith said.

Obama's spokesman Bill Burton shot back: "This is a transparent and laughable attempt to divert attention from the caucus results, which reward delegates every bit as meaningful as do primaries."

Heading into the make-or-break contests, Clinton said earlier in Houston that she was "feeling really optimistic".

Asked about her plans going forward, the New York senator said: "I believe in taking things one day at a time in politics, as in life."

Obama said Clinton had hurled the "kitchen sink" at him in recent days in eviscerating his credentials to be commander-in-chief and overseer of an economy headed for trouble.

Citing a leaked memorandum from the Canadian government, Clinton alleges that Obama is hood-winking voters with his vocal opposition to the North American Free Trade Area (NAFTA) - a charge angrily denied by his campaign.

And at a combative press conference Monday, Obama was grilled about his ties to a Chicago property developer, Antoin "Tony" Rezko, who is on trial for corruption.

For its part, Obama's campaign mocked Clinton's argument that her "35 years of experience" including her time as first lady qualified her to be president.

Clinton "failed the commander-in-chief test" by voting for the war in Iraq, and had made "exaggerated claims about her role in Kosovo and the Northern Ireland peace process," Burton said.

According to RealClearPolitics.com, Obama leads by 1,392 delegates to Clinton's 1,279.

The winning line to secure the White House nomination is 2,025.

A total of 370 Democratic delegates were at stake on the biggest day of primary voting since "Super Tuesday" on February 5.

But the Democratic Party apportions its delegates proportionally, so the overall tally was unlikely to change much either way. Instead battle lines would be drawn about who has the momentum now to take the fight to McCain.

The Republican's campaign was written off over the summer when he was rocked by an exodus of both cash and staffers.

But eight years after his failed bid against Bush, he will now take on the mantle against the Democrats in November with the promise of staying in Iraq as long as it takes to defeat the "transcendent" threat of Islamic extremism.