Hillary Clinton and Donald Trump traded blows on the debate stage Monday over the Democrat’s use of a private email server as secretary of state, and the Republican billionaire’s refusal to release his tax history.

Democrat Hillary Clinton accused Republican Donald Trump of racism, sexism and tax avoidance on Monday. Trump, a real estate tycoon making his first run for public office, said Clinton´s long years of service represented "bad experience" with few results and suggested her disavowal of a trade deal with Asian countries was insincere.

For Trump, 70, the debate was a chance to appear disciplined. For Clinton, 68, it was an opportunity to reassure voters she could be trusted.

It remained to be seen how voters would judge their performance. 

African-American voters overwhelmingly support Clinton, but Trump in recent weeks has said he believes his policy agenda would benefit them and said the policies of Obama and Clinton had failed to help black Americans.

He said Clinton´s arguments were disingenuous.

"When you try to act holier than thou, it really doesn´t work," Trump said.

RED AND BLUE

Clinton wore a red pantsuit, and Trump wore a dark suit and a blue tie to the encounter that could shift the course of the tight race for the Nov. 8 election.

Toward the end of the debate, Trump said Clinton did not have the endurance to be president.

"She doesn´t have the look, she doesn´t have the stamina," he said.

Citing her own public record, Clinton retorted: "As soon as he travels to 112 countries and negotiates a peace deal, a ceasefire, a release of dissidents ... or even spends 11 hours testifying in front of a congressional committee, he can talk to me about stamina.”

Clinton called the New York businessman´s tax policies "Trumped-up trickle-down" economics and Trump accused the former secretary of state of being "all talk, no action."

Clinton knocked Trump for not releasing his income tax returns and said that decision raised questions about whether he was as rich and charitable as he has said.

She noted that the few years of tax returns he had released showed that despite his wealth, he had paid no federal income tax.

"That makes me smart," Trump said. "I have a tremendous income," he said at one point, adding that it was about time that someone running the country knew something about money.

"I will release my tax returns against my lawyer’s wishes when she releases her 33,000 e-mails that have been deleted," Trump said.

Clinton shot back that such a challenge was "another example of bait and switch here," noting that presidential candidates for the last 40 years have released their tax returns.

"I have no reason to believe that he’s ever going to release his tax returns, because there’s something he’s hiding," Clinton said.

In December 2014, after a formal request by the State Department, Clinton turned over more than 30,000 emails. Months later, she announced that she had asked the department to make public many of those, while also saying that she deleted more than 32,000 other personal emails.

Trump and other Republicans have repeatedly argued that Clinton may have deleted emails in order to prevent information damaging to Clinton from becoming public.

"If I had to do it over again, I would, obviously, do it differently," Clinton acknowledged about her use of private email. "But I’m not going to make any excuses. It was a mistake and I take responsibility for that."

"That was not a mistake. That was done purposely," Trump interjected.

TRADE AT ISSUE

Trump attacked Clinton for her trade policies and said she would approve a controversial trade deal with Asian countries despite opposing it as a candidate.

"You were totally in favor of it, then you heard what I was saying, how bad it is, and you said, ´Well, I can´t win that debate,´ but you know that if you did win, you would approve that," he said.

Clinton rejected the criticism. "Well Donald, I know you live in your own reality, but that is not the facts," she said.

Moderator Lester Holt struggled to rein in the candidates, with discussions about trade policy suddenly shifting to the fight against Daesh as Trump accused Clinton of giving away information to the enemy by revealing on her website how she planned to defeat the group.

Clinton said that unlike Trump, she at least had a plan for fighting the militants.

Opinion polls have shown the two candidates in a very tight race, with the latest Reuters/Ipsos polling showing Clinton ahead by 4 percentage points, with 41 percent of likely voters.

A second Reuters/Ipsos poll released on Monday showed half of America´s likely voters would rely on the debates to help them make their choice.

Two other presidential candidates - Libertarian Gary Johnson and Green Party candidate Jill Stein - were not invited to take part in the debate because neither had obtained at least 15 percent support in national polls, the threshold established to qualify.