WASHINGTON (Reuters) - President Donald Trump on Wednesday signed a new North American trade agreement during an outdoor ceremony at the White House attended by about 400 guests - but not the key Democrats who helped secure congressional passage of the deal.

Trump, on trial in the U.S. Senate on charges of abusing power and obstructing Congress, welcomed Republican senators at the South Lawn event by name. Other guests included lawmakers from around the country, workers, farmers and chief executives, and officials from Mexico and Canada, the White House said.

The U.S.-Mexico-Canada Agreement (USMCA) will replace the 26-year-old North American Free Trade Agreement, including tougher rules on labor and automotive content but leaving $1.2 trillion in annual U.S.-Mexico-Canada trade flows largely unchanged.

“Today, we are finally ending the NAFTA nightmare and signing into law the brand-new U.S.-Mexico-Canada Agreement,” Trump told the crowd. Flanked by a group of American workers wearing hard hats, Trump said the agreement would bolster U.S. economic growth, benefiting farmers, workers and manufacturers.

He said his concerns about NAFTA-triggered outsourcing had triggered his run for the presidency in 2016.

A wide array of business groups welcomed the agreement, which also won a rare endorsement of the AFL-CIO union federation. Mexico has already approved the deal, but it must still be ratified by Canada’s parliament before it can take effect.

The influential United Autoworkers said the new deal would not restore hundreds of thousands of U.S. manufacturing jobs lost to Mexico after NAFTA, and vowed to be aggressive in pushing for enforcement of the trade agreement’s provisions.

Democratic presidential candidate Bernie Sanders, a Vermont senator who voted against the pact earlier this month, said the NAFTA replacement deal would not stop the offshoring of U.S. jobs and amounted to “a giveaway to the fossil fuel industry.” He said he would immediately renegotiate the pact if elected.

Canadian Prime Minister Justin Trudeau, speaking in Ottawa, said his minority government would continue to answer questions posed by various industries and other groups.

“We have questions and we have a process for ratification. I just look forward to getting, getting through it responsibly and rapidly because it’s so important for Canadians,” he said.

NO DEMOCRATS

Excluded from the event were House of Representatives Speaker Nancy Pelosi, House Ways and Means Committee Chairman Richard Neal and other Democrats who negotiated with the Trump administration for months to expand the pact’s labor, environmental and enforcement provisions and pave the way for its approval by the Democratic-controlled House.

Trump did not mention the work done by Pelosi or other Democrats on the trade pact, but U.S. Trade Representative Robert Lighthizer, in his remarks at the ceremony, acknowledged the role that House leaders played in getting the deal done.

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Partisan tensions were running high as U.S. senators started to pose questions in Trump’s impeachment trial, ahead of a key vote later this week on whether to allow the calling of witnesses like former national security adviser John Bolton.

The White House on Wednesday objected to the publication of a book written by Bolton that depicts Trump as playing a central role in a pressure campaign on Ukraine to launch investigations of Democrats, including former Vice President Joe Biden, a top contender for the Democratic nomination to face Trump in this year’s election.

Pelosi told reporters that Democrats had ensured “vast improvements” to the USMCA before it was approved, despite their absence from Trump’s White House event, adding, “I hope he understands what he’s signing today.”

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Neal told reporters some Republican senators thought the deal was “too Democratic.” He said the final accord won stronger protections for workers, better enforcement of environmental provisions and steps to prevent higher drug prices.

Representative Rosa DeLauro told reporters in a separate teleconference that Democrats would remain vigilant on oversight of the improved trade deal and would fight for even better climate protections in future trade deals.

The U.S. Senate this month overwhelmingly approved legislation to implement the USMCA, sending the measure to Trump for signing into law.

U.S. lawmakers said it was unclear when the accord would take effect, since Canada’s main opposition Conservative Party had expressed concerns about aspects of the deal and there was no exact timeline for ratification there.

Even after Canada ratifies the accord, implementation could take several more months since the three countries must show they are meeting their obligations before the clock starts ticking on an effective date.