Tom Udall, a Democrat from New Mexico, announced Monday he will not seek re-election to the U.S. Senate in 2020.
The two-term senator said in a statement that although he is confident he could win a third term, he will not run again next year because "the worst thing anyone in public office can do is believe the office belongs to them, rather than to the people they represent."
Udall's announcement creates uncertainty about a Democratic seat in the Senate, in which Democrats are already in the minority. However, a Republican hasn't won a Senate seat in New Mexico since the 2002 election of former Sen. Pete Domenici.
The top spokesman for the National Republican Senatorial Committee suggested that Udall was retiring because of the Democrats' "lurch toward socialism," an increasingly familiar line of attack ahead of 2020.
"The people of New Mexico deserve a Senator who will fight for more jobs, lower taxes, and safer communities and we look forward to fielding a strong candidate in the New Mexico Senate race," the NRSC said in a statement.
Republicans hold a 53-47 edge in the Senate, where there will be 34 seats up for grabs in 2020.
Udall said that without the distraction of another campaign he can now help "reverse the damage done to our planet, end the scourge of war, and to stop this president's assault on our democracy and our communities."
Udall said he isn't retiring from politics and that he plans to find new ways to serve New Mexico once he finishes his time in the Senate.
Senate Minority Leader Chuck Schumer said in a statement that Udall is "a class act in every way" and will be "greatly missed" in the Senate. Schumer said Udall's efforts during his time in the Senate have been "invaluable."
"He is principled, smart, and hardworking and when he gets up to speak, every senator – Democrat and Republican – already knows how well-thought-out and sincere his remarks will be," Schumer said.
Udall serves on five Senate committees: Appropriations, Foreign Relations, Commerce, Indian Affairs and Rules and Administration. Before he was elected to the Senate, he represented the 3rd Congressional District of New Mexico in the House of Representatives.
Udall's office did not immediately respond to CNBC's request for additional comment.