Senator Tammy Baldwin and US trade representative, Jamieson Greer. Al Drago | Graeme Sloan | Bloomberg | Getty Images A group of Democratic senators will issue a set of demands to U.S. Trade Representative Jamieson Greer ahead of a mandatory joint review of the U.S.-Mexico-Canada Agreement this summer. In a letter led by Sen. Tammy Baldwin, D-Wis., and shared exclusively with CNBC before being sent later Wednesday, 15 Democrats wrote to Greer to “insist that any revised agreement must deliver meaningful and measurable gains for American workers.” The USMCA, struck during President Donald Trump’s first term, is up for review on July 1. While initially touted by Trump as “the fairest, most balanced, and beneficial trade agreement we have ever signed into law,” the president has soured on the pact lately — slapping Mexico and Canada with tariffs during his second term. Greer has also, in testimony to Congress in December, said that “a rubber stamp of the Agreement is not in the national interest,” meaning that significant changes may be required to reapprove the …