Senator Tom Udall of New Mexico announced on Monday that he would retire at the end of his term, becoming the first Democratic senator to declare he would leave the chamber after the 2020 elections and setting in motion a competitive race to succeed him among a rising generation of Western Democrats.
Mr. Udall, 70, the scion of a political dynasty that helped define the modern Democratic Party in the West, explained his decision in an online message, writing that he believed he could be more productive over the next two years without the pressures of seeking re-election.
“Without the distraction of another campaign, I can get so much more done to 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,” Mr. Udall wrote on Medium.
First elected to the Senate in 2008, Mr. Udall previously served in the House and as New Mexico’s attorney general, following in the steps of family members who forged a path to power as liberal environmentalists well before demographic change made the Southwest a prime electoral battleground.
His father, Stewart Udall, represented Arizona in the House and served as secretary of the interior, while his uncle, Morris K. Udall, known as Mo, was a senior member of the House who ran for president. A cousin, Mark, was a senator from Colorado.
Tom Udall’s rise in elected office mirrored New Mexico’s emergence as a safely Democratic state. After voting for George W. Bush in 2004, the state has turned progressively bluer and currently has no Republicans in its congressional delegation. Mr. Udall won his Senate seat in 2008 after the retirement of Senator Pete V. Domenici, a Republican who was the last member of the party to represent New Mexico in the chamber.
Democrats are likely to have an advantage in the race for Mr. Udall’s seat, and it is not clear how seriously Republicans might target the race, given how many incumbent senators they must defend nationwide. President Trump’s unpopularity has taken a severe toll on Republicans in New Mexico and other diverse Western states, and it could be difficult for the party to mount a strong campaign there during a presidential election.
But the Democratic side of the race is likely to be freighted with significance in other ways. New Mexico was among the states that saw a tide of young and diverse Democratic officeholders sweep to power during last year’s midterm elections, and some of those newly elected officials may now consider running for Senate.
The state elected a Latina chief executive, Gov. Michelle Lujan Grisham, and sent two new women to the House: Representative Deb Haaland, 58, who became one of the first Native American women elected to Congress; and Representative Xochitl Torres Small, 34, a Latina former aide to Mr. Udall who carried a conservative-leaning district that hugs the border.
Most senior in New Mexico’s House delegation is Representative Ben Ray Luján, a former chairman of the Democratic Congressional Campaign Committee who now serves as deputy speaker of the House.
In addition, there are a number of ambitious Democrats in state and local offices who could join the Senate race, including Tim Keller, the mayor of Albuquerque, and Hector Balderas, the state attorney general who ran unsuccessfully for Senate in 2012.
There are a handful of prominent Republicans in the state, including Susana Martinez, the former governor, and Richard J. Berry, the former mayor of Albuquerque. It is unclear if any would be interested in mounting a long-shot campaign, and Ms. Martinez left office deeply unpopular.
Mr. Udall offered no hint as to a preferred heir on Monday, but he indicated that he intended to remain actively involved in politics and government. It has been speculated for some time that he could join the cabinet of a Democratic president, perhaps serving as interior secretary like his father.
“I’m most certainly not retiring,” Mr. Udall wrote. “I intend to find new ways to serve New Mexico and our country after I finish this term. There will be more chapters in my public service to do what needs to be done.”