President Trump’s secret weapon and son-in-law has struck again.

Insiders say that Jared Kushner was key to nailing down the revision to the North American Free Trade Agreement announced Monday, working with U.S. Trade Representative Robert Lighthizer, the Canadians, and especially Mexico's Foreign Minister Luis Videgaray.

“This is one of his most significant accomplishments,” said an administration official of Kushner.

Kushner, married to first daughter Ivanka Trump, has been a top domestic and foreign policy adviser to the president since the 2016 election. He has been involved in several major agenda victories for the White House, among them the push for prison reform, bringing jobs to the U.S. from overseas, working up a Middle East peace timeline, nailing down the 2028 Summer Olympics, and securing soccer’s World Cup in North America in 2026.

His focus on Mexico during the 2016 presidential campaign was one of the reasons he wanted to work inside the West Wing, said an official. It was Kushner, for example, who worked out the campaign year visit by President Trump to Mexico.

“He has a close relationship with Mexico. It is just natural that he was brought into the NAFTA and other talks,” said the official, who added that Kushner has been working with U.S. officials on NAFTA reforms for a year and a half.

Chris Garcia, former deputy director at the Commerce Department under Trump, said that Kushner has been an effective trade negotiator for the president.

In an email, he told Secrets, “Jared, as a trusted advisor to POTUS, has seen his influence grow in the NAFTA trade discussions because POTUS sees him as someone who will ensure the president's tough stance is represented at the table and monitor the talks to ensure they come to an amicable, but fair, resolution.”

Just this weekend, he and Lighthizer met with Mexican Economy Secretary Ildefonso Guajardo to finish the deal, one of Trump’s top priorities.

[Also read: Trump says China wants trade deal, but it's 'not the right time']