(Adds Democrats' comments)
By Jeff Mason and Andrea Shalal
WASHINGTON, Jan 29 (Reuters) - President Donald Trump will sign a new North American trade agreement on Wednesday in an outdoor ceremony at the White House to be attended by about 400 guests - but not the key Democrats who helped secure congressional passage of the deal.
Guests invited to the South Lawn signing include lawmakers from around the country, workers, farmers and CEOs, as well as officials from Mexico and Canada, a White House official said.
Not invited were House Ways and Means Committee Chairman Richard Neal and other Democrats who negotiated for months to expand the pact's labor, environmental and enforcement provisions and ensure the approval of the Democrat-controlled House, said sources familiar with the situation.
The event coincides with Trump's impeachment trial on charges of abusing power and obstructing Congress, amid signs that some of his fellow Republicans may allow the calling of witnesses like former National Security Adviser John Bolton.
Trump lashed out against Bolton on Twitter on Wednesday after Bolton wrote in an unpublished book manuscript that the president told him he wanted to freeze $391 million in security aid to Ukraine until Kiev pursued investigations of Democrats, including former Vice President Joe Biden, a top contender for the Democratic nomination to face Trump in this year's election.
Trump had made renegotiating the 26-year-old North American Free Trade Agreement (NAFTA) a major campaign issue in 2016 and plans to tout its replacement at events in coming weeks as he campaigns for a second term.
"Along with members of Congress, state and local leaders, and workers from across the country, including farmers, ranchers, and entrepreneurs, President Trump will celebrate another promise made, promise kept to the American people when he signs the U.S.-Mexico-Canada Agreement (USMCA)," White House spokesman Judd Deere said.
"USMCA rebalances trade in North America, replaces the job-killing NAFTA, ends the outsourcing of American jobs, and invests in the American worker," he said.
House Speaker Nancy Pelosi told reporters that Democrats had ensured "vast improvements" to USMCA before it was approved, even if they would not be present at the signing ceremony. "I hope he understands what he’s signing today," she said of Trump.
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 United States-Mexico-Canada Agreement must still be ratified by Canada before it can take effect. The U.S. Senate this month overwhelmingly approved the legislation, sending the measure to Trump for signing into law.
The Mexican parliament has already approved the deal.
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.
The USMCA includes tougher rules on labor and automotive content but leaves $1.2 trillion in annual U.S.-Mexico-Canada trade flows largely unchanged.
Reporting by Andrea Shalal and Jeff Mason; Additional reporting by David Shepardson; Editing by Clarence Fernandez and Dan Grebler