Twenty-one senators signed on to a letter Wednesday asking the Commerce Department to allow crude oil to be exported to Mexico.

The bipartisan group led by Sens. Lisa Murkowski Lisa Ann MurkowskiSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (R-Alaska) and Heidi Heitkamp Mary (Heidi) Kathryn HeitkampCentrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents Susan Collins set to play pivotal role in impeachment drama MORE (D-N.D.) said Commerce Secretary Penny Pritzker should allow exports to Mexico in the same way Canadian exports have been allowed since the 1980s.

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They asked Pritzker to allow an oil swap that Mexican state-owned oil company Pemex recently requested, and to go further. The swap would exchange heavy crude from Mexico for light crude from the United States.

“In fact, we believe it would be appropriate to further liberalize energy trading with Mexico,” they wrote.

“President Reagan issued a national interest finding in 1985 stating that oil exports to Canada (for consumption in that country) were in accord with existing statutes and would not threaten U.S. supply,” they said.

“As a result of the expressed interest from Mexico in obtaining U.S. crude oil, we encourage the current administration to follow President Reagan’s example by issuing a similar finding that United States oil exports to Mexico, for consumption in Mexico, are in the national interest.”

Crude oil exports have been all but banned for 40 years, thanks to a law passed after the 1973 oil crisis. Some lawmakers have advocated recently for Congress to loosen the ban, and for the Obama administration to use whatever authority it has to allow some exports.

Last year, Commerce started to allow condensate, a lightly processed form of oil, to be exported.