Under USMCA, if the controversy is not resolved during consultations, a dispute panel can be called to adjudicate.
Lopez Obrador has put on a bullish front, saying Mexico has broken no laws and that "nothing is going to happen."
Resolution appears to hinge on whether energy nationalists inside the Mexican administration, who have taken their cues from Lopez Obrador, are prepared to compromise.
Lopez Obrador has made energy policy a cornerstone of his presidency, making it hard for him to back down.
Still, the spat has hit investor confidence in Mexico, and Lopez Obrador is seeking U.S. help to finance solar power output in northern Mexico and attract investment in greener manufacturing, particularly in carmaking, a key industry.