New Mexico U.S. Sen. Tom Udall announced Monday he will not seek re-election in 2020, opening a secure Democratic seat to competition and ending a chapter in a Western political dynasty built on environmental conservation.

The second-term senator and former congressman said he was confident he could run a strong campaign but preferred to look for new ways to serve the public, without specifying how.

The decision marks an end to a 20-year political career on Capitol Hill for Udall, who first was elected to Congress in 1998.

It also puts a bookend on the enduring influence in Washington of the Udall clan, once referred to as the “Kennedys of the West.”

Udall’s father, Stewart Udall, served as Interior Secretary in the 1960s under Presidents John F. Kennedy and Lyndon B. Johnson and helped write far-reaching conservation legislation.

His uncle was Morris “Mo” Udall, a longtime Arizona congressman and a prominent Democratic 1976 presidential contender. Cousin Mark Udall of Colorado became a one-term U.S. senator.

Udall took up the family political mantle in 1999 as he joined Congress to  represent New Mexico’s sprawling northern district.

The 70-year-old senator from Santa Fe said he’ll dedicate the final two years of his term to fighting climate change, protecting public lands and to trying “to stop the president’s assault on our Democracy and our communities.”

“The worst thing anyone in public office can do is believe the office belongs to them,” Udall said in a videotaped message. “There will be more chapters in my public service to do what needs to be done.”

In 2008, Udall easily defeated then-U.S. Rep. Steve Pearce to take Democratic control of a Senate seat held for 36 years by Pete Domenici, who retired when diagnosed with a degenerative brain disease and died in 2017.

In one crowning legislative achievement, Udall ushered through Congress with scant opposition a modernization of outdated chemical safety regulations that were signed by President Barack Obama in 2016. He grew worried that the law would be misused to protect industry under the administration of President Donald Trump.

Since 2017, Udall has helped organize resistance in Congress to attempts by the Trump administration to roll back development restrictions on public lands, while defending the regulation of heat-trapping gasses linked to climate change.

“He has been one of the main leaders and architects in the Senate of all the efforts to defend against the rollback of regulations under Trump,” said Jon Goldstein, director or regulatory and legislative affairs for the Environmental Defense Fund, noting Udall’s defense in particular of rules to reduce the release or burning of excess methane by industry. “He really quarterbacked the effort.”

New Mexico’s junior senator, Democrat Martin Heinrich, last year won re-election, easily defeating former Libertarian New Mexico Gov. Gary Johnson and Republican political newcomer Mick Rich.

Democrats consolidated control of New Mexico’s delegation to Washington in November elections, when U.S. Rep. Xochitl Torres Small flipped a district in the south of the state that was long dominated by the GOP.

Democratic colleagues in Washington and serving in statewide office rushed to praise Udall for his record on environmental conservation and the protection of public lands – a Udall family trademark – and as an advocate for Native American communities that span New Mexico.

“New Mexico will lose a powerful advocate,” said Sen. Dianne Feinstein of California, praising Udall’s work on renewable energy and tribal land issues.

Udall used the announcement of his approaching retirement to sound an alarm about the threat of climate change, the influence of anonymous spending on politics and the potential for new armed conflicts without the authorization of Congress.

Born in Tucson, Arizona, Udall hails from a ranching family that once drove cattle across territorial New Mexico.

He studied law and got off to a shaky start in politics, losing two initial bids for Congress. He served two terms as state attorney general before defeating an incumbent Republican Congressman in 1998.

Defining votes in Congress and Senate include opposition in 2002 to war in Iraq and his rejection of federal bailout packages designed to save the nation’s economy after the Great Recession.

His work on behalf of Native American tribal communities across New Mexico included recent efforts at stricter laws against the sale of Indian cultural artifacts and fraudulent indigenous art. He is seeking a permanent land buffer to keep oil and gas drilling away from Chaco Culture National Historical Park and other sites held sacred by Native American tribes.

It was initially unclear whether Torres Small or two other members of New Mexico’s U.S. House delegation – also Democrats – might compete for Udall’s vacant seat.

Rep. Ben Ray Lujan, 46, is a member of House Democratic leadership, and first-term Congresswomen Debra Haaland of Albuquerque fended off a crowded field of Democrats in last year’s primary.

A Lujan spokeswoman declined to comment on whether he would run for Senate. Haaland and Torres Small aides did not immediately return messages left for them.

New Mexico Republicans are scarce in prominent public offices, having been driven out of every stateside elected office in the 2018 elections. Democrats serve as mayor in three of the state’s four largest cities – Albuquerque, Santa Fe and Las Cruces.

Udall was spending the day with family in Santa Fe after making the announcement.