Hillary Clinton said Thursday she would defend US interests against China and reject the proposed Pacific trade agreement, as she struck a tough tone on global economic issues.

Clinton was speaking to a crowd in Warren, Michigan, a suburb of Detroit, just three days after presidential rival Donald Trump laid out his own economic plan, and she sought to highlight their fundamental differences.

The former secretary of state, leading in national polls with less than three months before the election, also sought to expose the "myth" that New York billionaire Trump would punish the rich and side with working- and middle-class Americans when it comes to economic policy.

"Now, there is a myth out there that he will stick it to the rich and powerful because somehow he's really on the side of the little guy," she told the crowd in Warren.

"Don't believe it," she said.

"He would give trillions in tax cuts to big corporations, millionaires and Wall Street money managers," she added. "That would explode our national debt and eventually lead to massive cuts in priorities like education, health care, and environmental protection."

Trump's opposition to trade deals like the Trans-Pacific Partnership (TPP) and the North American Free Trade Agreement (NAFTA) has become a main policy platform for the real estate tycoon, and he has sought to win support from American workers badly impacted by globalization, painting Clinton as a reckless backer of such agreements.

Clinton said protecting US interests did not require that Americans "cut ourselves off from the world," but she stressed that she would oppose trade pacts that weaken America's standing.

"My message to every worker in Michigan and across America is this: I will stop any trade deal that kills jobs or holds down wages, including the Trans-Pacific Partnership," she said.

"I oppose it now, I'll oppose it after the election, and I'll oppose it as president."

Clinton's position on trade is a vulnerability in the election. In 1993, her husband Bill Clinton signed NAFTA into law. It became very unpopular in some communities as companies relocated to Mexico to take advantage of cheaper labor, a result Trump routinely highlights at campaign stops.

While Hillary Clinton came out late last year against the TPP negotiated by Obama and 11 other countries, many critics including Trump have hit her for having promoted the pact, during the negotiation phase, as the "gold standard" for trade deals.

In Michigan, she adopted a firm tone, pledging to get tough with Beijing and saying the world's second largest economy has "gamed the system" for too long.

"I will stand up to China and anyone else who tries to take advantage of American workers and companies," she said. And when nations break the rules, "we won't hesitate to impose targeted tariffs."

Trump's campaign blasted Clinton before she had even finished speaking.

"Make no mistake: she will approve TPP, just like she pushed through the Korea and the Colombia deals," Trump's national policy director Stephen Miller said in a statement.