Lawmakers on both sides of the aisle are pushing the Trump administration to reject an appeal from Puerto Rico to temporarily waive a controversial measure called the Jones Act, a law intended to protect American shipping and maritime interests.

The Jones Act was adopted in 1920 and stipulates that only U.S. ships can carry cargo between U.S. ports, an attempt to strengthen the merchant marine fleet by nixing foreign competition. Similarly, it requires that at least 75 percent of the ship’s crew be American citizens and that the vessels be built or rebuilt in the U.S.