AFP/special correspondent

Fairfax, US

Americans began voting Tuesday in what is deemed the most pivotal day in the presidential nominating process, with frontrunners Hillary Clinton and Donald Trump hoping to wipe out their rivals.

Voters in a dozen states will take part in "Super Tuesday" - a series of primaries and caucuses in states ranging from Alaska to Virginia, with Virginia the first to open its polling stations at 6:00 am (1100 GMT).

If Democrat Clinton and Republican Trump - an outspoken billionaire political neophyte who has unexpectedly tapped into a vein of conservative rage at conventional politics - win big, it could spell doom for their challengers.

Hours before polls opened, the duo made last-ditch appeals to supporters ahead of a day like few others on the calendar leading up to the November election for the White House. Trump's Republican rivals, Senators Marco Rubio and Ted Cruz, were frantically trying to halt the real estate magnate's march toward nomination, seeking to unite the party against the man they see as a non-conservative political interloper.

Clinton meanwhile was riding high after thrashing rival Bernie Sanders in South Carolina over the weekend, securing an astronomical 86 percent of the African-American vote in her third win in four contests.

Should she win black voters by similar margins in places like Alabama, Georgia and Virginia, she should dominate there to become once again the inevitable candidate.

That was her status at the start of the campaign - before the rise of Sanders, a self-described democratic socialist. She was leaving nothing to chance, traveling to multiple states on Monday to urge a strong turnout.

Clinton also took aim at the increasingly hostile campaign rhetoric on the Republican side led by Trump.

"I really regret the language being used by Republicans. Scapegoating people, finger-pointing, blaming. That is not how we should behave toward one another," she told hundreds gathered at a university in Fairfax, Virginia.

"We're going to demonstrate, starting tomorrow on Super Tuesday, there's a different path that Americans ought to take." Trump's incendiary campaign has infuriated Republican rivals, including mainstream favorite Rubio who has intensified his personal attacks and stressed Trump would have trouble in a general election.

The Florida senator warned supporters in Tennessee that US media and Democratic groups will jump on Trump "like the hounds of hell" if he wins the nomination.

But Trump is clearly in the driver's seat. He is leading in polls in at least eight of the 11 Super Tuesday states.

Meanwhile, democrats Hillary Clinton and Bernie Sanders would both defeat Republican presidential front-runner Donald Trump in a general election, according to a new poll. Clinton tops Trump in a hypothetical matchup, 52 to 44 percent, in the CNN/ORC survey released Tuesday. Sanders, a self-proclaimed socialist, would also beat the boisterous billionaire businessman, pollsters found winning by a 55-to-43-percent margin.

Trump has closed the gap with Sanders while Clinton has gained ground against the businessman, pollsters found. Trump and Clinton were tied with 48 percent apiece during a similar sampling last September, CNN/ORC reported. Sanders, meanwhile, has slipped from his best showing last July, when he would have won by a 59-to-38-percent margin. Clinton is the underdog against Republican Senators Marco Rubio and Ted Cruz in other theoretical pairings in the new poll. Rubio tops Clinton, 50 to 47 percent, while Cruz edges past her, 49 to 48 percent. Sanders fares more favourably against the pair of Republican White House hopefuls, however.

He takes 53 percent to Rubio’s 45 percent, and he bests Cruz in a hypothetical matchup, 57 to 40 percent.

CNN/ORC polled 920 registered voters from Feb. 24-27. Their survey has a 3-percent margin of error. Clinton leads Sanders by about 10 points nationwide, according to the latest Real Clear Politics average of polls. Trump, meanwhile, has a nearly 16-point edge over the Republican presidential field in the latest edition of the same index. Voters in 11 states and American Samoa are casting ballots today, making it the single biggest day of the 2016 election cycle so far.