Sen. Tom Udall (D-N.M.) a friend of the Los Alamos and Sandia National Laboratories who has spent the last two decades on Capitol Hill, on Monday announced he will not run for reelection in 2020.

Udall is serving his second term in the Senate and will leave the body in January 2021. Since 2009, Udall has occupied the Senate seat once held by Peter Domenici: the Republican known by some of his nuclear constituents in New Mexico — whose interests in Washington he tended carefully — as St. Pete.

Before winning his Senate seat, Udall was the U.S. representative for New Mexico’s third district. He represented the district, which includes the Los alamos National Laboratory, from 1999 to 2009. Before the House, Udall was New Mexico’s attorney general from 1991 to 1999, during which time Congress passed the 1992 Land Withdrawal Act that created the nation’s only permanent, deep-underground transuranic waste disposal facility, the Waste Isolation Pilot Plant.

In his second, and now to be final, Senate term, Udall pushed back against the DoE National Nuclear Security Administration’s (NNSA) plans to move part of the agency’s capacity to produce fissile nuclear-weapon cores called plutonium pits away from Los Alamos.

“There is only one place in the U.S. with the technical know-how to do [pits], and that is Los Alamos National Laboratory,” Udall told Energy Secretary Rick Perry and NNSA Administrator Lisa Gordon-Hagerty in a budget hearing last year.

Udall has objected to NNSA’s plans to build a pit plant at the Savannah River Site in Aiken, S.C., saying the agency is ignoring an upgrade previously planned at Los Alamos capable of stamping out the 80 pits a year the Trump administration wants NNSA to produce by 2030.

Udall has also stuck up for the independence of the Defense Nuclear Facilities Safety Board: the federal health-and-safety watchdog for active and former DoE nuclear weapons sites. In a September letter co-signed by his colleague, Sen. Martin Heinrich (D-N.M.), Udall asked DoE to reconsider the rule it published last year that curtails some agency interactions with the defense board, and which the board feared could limit its access to active and former weapon sites.