A group of 21 U.S. senators called on the Commerce Department to liberalize trade relations and allow for greater crude oil trade with Mexico.

U.S. Sen. Lisa Murkowski, R-Alaska, the head of the Senate Energy and Natural Resources Committee, was joined by 19 other Republicans and one Democrat, North Dakota Sen. Heidi Heitkamp, in issuing a request to Commerce Secretary Penny Pritzker.

"It's time to harmonize North America's energy trade," Murkowski said in a statement.

At issue is a request from state-owned Petroleos Mexicanos, known also as Pemex, to import lighter blends of U.S. crude oil and an ultra-light form of oil called condensate, found largely in shale, in exchange for the heavier Mexican grade. Pemex said it wanted the U.S. oil to blend with Mexican crude oil at its refineries.

The senators in their Wednesday letter to the Commerce Department argued Presidents Gerald Ford, Jimmy Carter and Ronald Reagan supported similar exports with Canada between 1976-85. Reagan in 1985 used the power of his office to allow oil exports to Canada and the 21 senators said Mexico deserves the same consideration.

Heitkamp said she met in 2014 with Pemex officials to discuss bilateral trade issues and would continue to work to eliminate what she said were "unnecessary burdens that block our global energy development."

Murkowksi and many of her allies on Capitol Hill have pressed for an end to a 1970s ban on exports of crude oil produced in the United States. By easing the ban, they've argued, consumers will benefit from lower fuel costs while the United States increases its trade leverage overseas.

A February paper from Columbia University's Center on Global Energy Policy said Mexican oil swaps would only make economic sense under certain crude oil market scenarios, which don't exist in the weak current market.