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In their first face-off just weeks before one of them becomes U.S. president, GOP nominee Donald Trump and Democratic rival Hillary Clinton sparred over trade agreements, with Trump frequently assailing U.S. companies that have taken production to Mexico.

In Monday's debate, held at Hofstra University in New York, Trump blamed Clinton for what he says have been "defective" trade agreements that have cost American jobs.

Trump said Mexico taxes American products imported there, but the U.S. does not tax Mexican imports.

“Our jobs are fleeing the country, they’re going to Mexico. Thousands of jobs are leaving Michigan, leaving Ohio,” Trump said. “We have to stop our jobs from being stolen from us…We have to stop our companies from leaving the United States and firing the American people.”

He says Clinton's been "doing this for 30 years," a reference to her long career on the American political scene.

Clinton responded that she held jobs, such as senator from New York and secretary of state, that did not give her a primary role in crafting trade agreements.

Trump also criticized the North American Free Trade Agreement, which was approved under President Bill Clinton, Hillary Clinton's husband.

The nonpartisan Congressional Research Service has said that 1994 deal had a "relatively small" impact on the U.S. economy.

For Clinton and Trump, the first of three debates was a crucial chance to boost their standing with voters who view both candidates negatively.

The televised face-off was the most hotly anticipated moment in an election campaign that has been both historic and unpredictable.

While immigration has been a focus of Trump’s campaign, the candidates were not asked about it by moderator Lester Holt of NBC.

Trump remarked that the union of immigration agents had endorsed him.

"It’s surprising that immigration was totally absent from today’s debate given that it’s one of the only issues that Trump has offered policy options and they are very different from Secretary Clinton’s," said Angela Kelley, senior vice president of the Center for American Progress, to Fox News Latino. "I hope that immigration gets time at the next debate."

Clinton emphatically accused Trump of purposely keeping his personal tax returns and business dealings secret from voters, declaring during the debate, "There's something he's hiding."

Trump aggressively tried to turn the openness questions around on Clinton, who is seen by many voters as secretive. Trump said he would release his tax information when she produces more than 30,000 emails that were deleted from the personal internet server she used as secretary of state.

The Republican has repeatedly said he can't release his taxes because he is under a routine audit.

Tax experts have said there is no reason the businessman cannot not make his tax records public during an audit.

Clinton was contrite in addressing her controversial email use, saying simply that it was a "mistake." She notably did not fall back on many of the excuses she has often used for failing to use a government email during her four years as secretary of state.

"If I had to do it over again, I would obviously do it differently," she said.

Trump countered, "That was more than a mistake."

Clinton also cast Trump as sexist, saying: "One of the worst things he said was about a woman in a beauty contest.”

Referring to former Miss Universe Alicia Machado, Clinton said Trump had called the former beauty queen "Miss Piggy" and "Miss Housekeeping" because she is Latina.

Machado, who was born in Venezuela, recently became a U.S. citizen.

“And you can bet she’s going to vote this November," Clinton said.

Clinton was measured and restrained as Trump repeatedly spoke over her and tried to interrupt her. A businessman and political novice, Trump continuously cast her as a "typical politician."

The 90-minute televised debate came six weeks before Election Day and with early voting already getting underway in some states. Despite Clinton's advantages, including a sophisticated get-out-the-vote operation and a favorable electoral map, the race is exceedingly close.

The candidates also tangled over taxes and how to bring good-paying jobs back to the United States.

Democrat Clinton said her Republican rival was promoting a "Trumped-up" version of trickle-down economics — a philosophy focused on tax cuts for the wealthy. She called for increasing the federal minimum wage, spending more on infrastructure projects and guaranteeing equal pay for women.

The candidates' families filed into the debate hall at New York's Hofstra University moments before Clinton and Trump took the stage. Their spouses, former President Bill Clinton and Melania Trump, shook hands before taking their seats.

Clinton aides spent the days leading up to the debate appealing for the media and voters to hold Trump to a higher standard than they believe he has faced for much of the campaign. Their concern is that if the sometimes-bombastic Trump manages to keep his cool onstage, he'll be rewarded — even if he fails to flesh out policy specifics or doesn't tell the truth about his record and past statements.

Trump's campaign has accused Clinton of trying to pre-determine the outcome and said their concerns reflected worries about the Democrat's debating skills.

Clinton, a former senator and secretary of state, is banking on voters seeing her as a steady hand who can build on the record of President Barack Obama, whose popularity is rising as he winds down his second term in office. She's called for expanding Obama's executive orders if Congress won't pass legislation to overhaul the nation's immigration system and for broader gun control measures. Overseas, she's called for a no-fly zone in Syria but has vowed to keep the military out of a large-scale ground war to defeat the Islamic State group.

For Clinton, victory in November largely hinges on rallying the same young and diverse coalition that elected Obama but has yet to fully embrace her.

Trump has tapped into deep anxieties among some Americans, particularly white, working-class voters who feel left behind in a changing economy and diversifying nation. While the real estate mogul lacks the experience Americans have traditionally sought in a commander in chief, he's banking on frustration with career politicians and disdain for Clinton to push him over the top on Election Day.

The Associated Press contributed to this report.

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