The U.S. House of Representatives overwhelmingly approved a new North American trade deal on Thursday that includes tougher labour and automotive content rules but leaves $1.2 trillion US in annual Canada-U.S.-Mexico trade flows largely unchanged.

The House passed legislation to implement the Canada-U.S.-Mexico Agreement (CUSMA) on trade 385-41, with 38 Democrats, two Republicans and one independent member voting no.

The bipartisan vote contrasted sharply with Wednesday night's Democrat-only vote to impeach U.S. President Donald Trump.

The House vote sends the measure to the Senate, but it is unclear when the Republican-controlled chamber will take it up. Senate Republican leader Mitch McConnell has said that consideration of the measure would likely follow an impeachment trial in the Senate, expected in January.

The CUSMA trade pact, first agreed upon in September 2018, will replace the 1994 North American Free Trade Agreement (NAFTA). Trump vowed for years to quit or renegotiate NAFTA, which he blames for the loss of millions of U.S. factory jobs to low-wage Mexico.

The U.S. House of Representatives’ passage of the <a href="https://twitter.com/hashtag/USMCA?src=hash&ref_src=twsrc%5Etfw">#USMCA</a> is a historic milestone in <a href="https://twitter.com/RealDonalTrump?ref_src=twsrc%5Etfw">@realDonalTrump</a>’s effort to modernize our trade relations. This agreement with <a href="https://twitter.com/hashtag/Mexico?src=hash&ref_src=twsrc%5Etfw">#Mexico</a> and <a href="https://twitter.com/hashtag/Canada?src=hash&ref_src=twsrc%5Etfw">#Canada</a> will allow us to work more effectively to achieve economic prosperity for our nations. —@SecPompeo

House Speaker Nancy Pelosi gave CUSMA a green light last week after striking a deal with the Trump administration, Canada and Mexico to strengthen labour enforcement provisions and eliminate some drug patent protections.

Pelosi said she was not concerned about Democrats handing Trump a political victory on CUSMA as they are trying to remove him from office.

"It would be a collateral benefit if we can come together to support America's working families, and if the president wants to take credit, so be it," Pelosi said during House floor debate before the vote.

Secretary of State Mike Pompeo said on Twitter that the CUSMA was a "historic milestone" in Trump's effort to modernize trade relations.

The changes negotiated by Democrats, which include tighter environmental rules, will also set up a mechanism to quickly investigate labour rights abuses at Mexican factories. They have earned the support of several U.S. labour unions that have opposed NAFTA for decades.

U.S. trade representative Robert Lighthizer made a concession by dropping a requirement for 10 years of data exclusivity for biologic drugs, a provision that Democrats feared would keep drug prices high and that they called a "giveaway" to big drugmakers.

Some of the most ardent trade skeptics in Congress have voiced support of the deal, including Rep. Debbie Dingell, who represents an autoworker-heavy district in southeastern Michigan. Dingell said in television interviews that she backed the bill, even though she was skeptical it would bring auto jobs back to Michigan. Rep. Ron Kind, a pro-trade Democrat from Wisconsin, one of the top dairy-producing states, praised new access to Canada's closed dairy market under CUSMA.

"A no vote is a return to the failed policy of the old NAFTA, the status quo, rather than this more modernized version," Kind said in floor debate.

Democrats and U.S. President Donald Trump's administration have for months been negotiating over the finer details of CUSMA. (Drew Angerer/Getty Images)

The agreement modernizes NAFTA, adding language that preserves the U.S. model for internet, digital services and e-commerce development, industries that did not exist when NAFTA was negotiated in the early 1990s. It eliminates some food safety barriers to U.S. farm products and contains language prohibiting currency manipulation for the first time in a trade agreement.

But the biggest changes require increased North American content in cars and trucks built in the region, to 75 from 62.5 per cent in NAFTA, with new mandates to use North American steel and aluminum.

In addition, 40 to 45 per cent of vehicle content must come from high-wage areas paying more than $16 an hour — namely the United States and Canada. Some vehicles assembled in Mexico mainly with components from Mexico and outside the region may not qualify for U.S. tariff-free access.

The U.S. Congressional Budget Office estimated earlier this week that automakers will pay nearly $3 billion more in tariffs over the next decade for cars and parts that will not meet the higher regional content rules.