The protagonists are set: Hillary Clinton versus Donald Trump. The battleground? Fifty states. Voters will now be treated to a historic, bruising spectacle as two of America's most polarizing figures wage war for the White House.

On the Democratic side is a candidate rich with political experience, a pioneering if controversial stateswoman embracing her chance to become the nation's first female commander-in-chief.

The Republican standing in her way is the brash billionaire businessman and political neophyte who rewrote the campaign playbook as he vanquished his many rivals for the nomination with a mudslinging unparalleled in modern American politics.

The ferocity of the primaries -- Americans and many around the world had never seen a modern US presidential race play out with such insulting and denigrating attacks -- promises to be just a preview of what is in store for the general election.

Clinton, the former secretary of state, euphorically seized the Democratic Party mantle Tuesday, seizing the nomination in dramatic fashion with primary wins in four of six states including the grand prize, California.

On perhaps the biggest night of her political career, as she declared a historic milestone for women, she assailed Trump for his divisiveness and his trademark slogan, "Make America Great Again."

"That's code for let's take America backward," she said, "back to a time when opportunity and dignity were reserved for some, not all."

"Don't let anyone tell you that great things can't happen in America," she said.

Trump was pledging a tough fight, choosing to lay into Clinton in his own presumptive victory speech and accusing her and her former president husband Bill of enriching themselves by "selling access, selling favors, selling government contracts."

And he signaled there would be no honeymoon, announcing a "major speech" for early next week in which he will address "all of the things that have taken place with the Clintons."


'No out-Trumping Trump'

The trajectory of the race is unlikely to soar into loftier issues.

Norman Ornstein, a resident scholar at the American Enterprise Institute, feared the race will be less a battle over substance like the economy or foreign policy than a clash of two outsized personalities.

"Clinton has deep knowledge and detailed platforms, but because Trump doesn't have any, nobody focuses on the differences," Ornstein told AFP in an interview.

"And because we have two candidates who are more distrusted than trusted, more disliked than liked, this becomes a race to the bottom."

Clinton will be looking to exploit the various controversies ensnaring Trump -- most recently his criticism of a Mexican-American federal judge, which sparked uproar within his own party -- as a way to enhance her own resume.

"There's no out-Trumping Trump, he is an extremely talented unique character," said Tim Miller, a former communications adviser to Republican Jeb Bush, one of the many candidates defeated by Trump in the primaries.

"Getting down in a mud-wrestling fight with him is going to be a loser. She has to focus on the issues that drive her base" of minorities and women and inspire young voters, Miller said.


'Temperamentally unfit'

But she will also need to make a compelling case for supporters of her Democratic rival, Bernie Sanders, to embrace her campaign to defeat Trump in November.

Trump, beating Clinton to the punch Tuesday, extended an olive branch to Sanders' supporters, saying: "We welcome you with open arms."

Sanders refused to capitulate Tuesday, vowing to "continue the fight" to the final Democratic primary in the capital Washington next week, and then on to the Democratic convention in July.

But the Vermont senator meets on Thursday with President Barack Obama -- perhaps a sign that Obama will play a significant role in uniting a deeply divided Democratic party to battle the 69-year-old real estate mogul.

Obama is soon expected to offer a formal endorsement of the 68-year-old Clinton, perhaps serving to coax hardline "Bernie or bust" fans back into the party tent.

Clinton has appeared completely dialed in to combatting Trump, especially since last week in San Diego when she earned plaudits from Democrats for her blistering critique of Trump as "temperamentally unfit" to be president, a line of attack she repeated Tuesday night.

Howard Dean, the former Vermont governor who ran for president in 2004, said he was impressed with Clinton's historic achievement.

"Any time a glass ceiling is shattered, it's important," he told AFP.

But he was quick to note that Clinton is capable of turning tough in an instant.

"If Hillary Clinton continues to campaign the way she did last week in San Diego, I think Trump is toast," he said.