From the Overview: "The proposed U.S.-Mexico-Canada Agreement (USMCA), signed in late 2018, requires implementing legislation that must be approved by Congress before it can enter into force. [...] Labor issues were a major congressional concern throughout the USMCA negotiations and remained an issue after the three countries concluded the agreement in September 2018. In response, the U.S. Trade Representative (USTR) and some Members of the House of Representatives negotiated changes to the USMCA; most of the changes were dedicated to labor modifications. USTR then negotiated the amendments with Canada and Mexico."
CRS In Focus, IF11308
Congressional Research Service: https://crsreports.congress.gov/