WASHINGTON — In a step toward a potential 2020 White House bid, Sen. Elizabeth Warren, D-Mass., on Thursday delivered a blistering critique of President Donald Trump's foreign policy, accusing him of emulating dictators and emboldening white supremacists.

The speech by Warren comes as Trump heads to Buenos Aires for a Group of 20 summit where he had been scheduled to meet one on one with Russian President Vladimir Putin — before Trump abruptly called off the meeting, citing Russia's seizure of Ukrainian ships and sailors in the Black Sea.

"We must face reality head on: President Trump's actions and instincts align with those of authoritarian regimes around the globe," Warren said in the speech at American University in Washington. "He cozies up to white nationalists. He undermines the free press and incites violence against journalists. He attacks the independence of our judiciary. He wraps himself in the flag and co-opts the military for partisan purposes — but he can't be bothered to visit our troops in harm's way."

She blamed Republicans in Washington for backing Trump at the expense of "fundamental American values." And she prompted applause from the crowd when she declared, "The time for holding back is over."

"Americans must demonstrate to this president and to the world that we are not sliding toward autocracy — not without a fight," she said.

Warren has not announced whether she plans to run for president in 2020. But she and Trump have publicly sparred on issues, from Trump's economic policy and still-unreleased tax returns to disproved allegations that Warren's hiring at Harvard Law School was due to her claim to Native American heritage.

Warren launched a new critique of Trump in her Thursday speech, pledging to oppose his new United States-Mexico-Canada Agreement, or USMCA, until he "reopens the agreement and produces a better deal for America's working families."

"As it's currently written, Trump's deal won't stop the serious and ongoing harm NAFTA causes for American workers," Warren said. "It won't stop outsourcing, it won't raise wages and it won't create jobs. It's NAFTA 2.0."

The deal is expected to be signed Friday by the leaders of Canada and Mexico on the sidelines of the G-20 meeting but must still be ratified by each country's legislature.

Warren also devoted some of her remarks to domestic issues. She called for leaders to "speak plainly" about incidents of homegrown terrorism in the United States, such as last month's Pittsburgh synagogue shooting, the deadliest domestic attack on Jews in U.S. history, as well as the attacks in San Bernardino, California; Charleston, South Carolina; Orlando, Florida; Charlottesville, Virginia; and Fort Hood in Texas.

"Just like the hateful terrorism of al-Qaida and ISIS, domestic right-wing terrorism is completely incompatible with our American values," Warren said, using a different name for the Islamic State. "It is a threat to American safety and security, and we must not tolerate it in the United States of America."

Michael Ahrens, a spokesman for the Republican National Committee, responded to Warren's speech in a statement Thursday evening.

"It's ironic that Elizabeth Warren has chosen to launch her 2020 campaign with the two topics she knows the least about: her heritage and foreign policy," Ahrens said. Warren did not talk about her heritage in her speech; Ahrens appeared to have been referring to Warren's release of a DNA test last month that suggested a lineage to a distant Native American ancestor.

The Post's David Weigel and Matt Viser contributed to this report.