Sen. Jeff Merkley (D-Ore.) will seek reelection in the Senate in 2020, he announced Tuesday. (Jose Luis Magana/AP)

Sen. Jeff Merkley (D-Ore.) has decided not to run for president, declining to join a crowded race that includes a half-dozen of his Senate colleagues.

Merkley announced his decision to supporters Tuesday morning. Instead of vying for the White House, he will seek a third term in the Senate in 2020.

“I’ve got this limited number of years of life on this planet — how can I have the biggest impact?” Merkley said in an interview. “That’s what I’ve been weighing. And I’ve reached the conclusion that the biggest impact I can have is here in the Senate.”

The 62-year-old senator would have started the presidential contest as a long shot. Outside of his home state, he is not very well-known. He would have faced the daunting task of introducing himself to a broad audience in a crowded field featuring other candidates who have spent weeks or months campaigning.

Merkley, who is one of the Senate’s most liberal members, identified three “mega crises” he wants to help address: a “crisis of our democracy,” rooted in voting and campaign finance laws; an “opportunity crisis” for families to thrive; and “climate chaos.”

The Oregonian was the only senator to endorse Sen. Bernie Sanders (I-Vt.) in the 2016 Democratic primary against Hillary Clinton, the eventual nominee. But Sanders will not automatically have his support this time, Merkley said.

Who is hoping to challenge Trump for president in 2020?

“I don’t have any plans to endorse anybody for a good length of time,” he said. Merkley said he hopes to encourage the Democratic presidential field to take on the three crises he identified.

Merkley will be heavily favored to retain his Senate seat in 2020. His is one of 12 seats Democrats are defending, compared with 22 for Republicans. The nonpartisan Cook Political Report has regarded the Oregon seat as solidly Democratic.

The Oregon Democrat was first elected to the Senate in 2008 and reelected to a second term in 2014. Before joining Congress, he served in the Oregon legislature, climbing the ranks to become speaker of the state House in 2007.

In the Senate, Merkley is a co-sponsor of the Green New Deal, a sweeping climate resolution championed by the left and strongly opposed by conservatives. He is also a co-sponsor of a bill to establish a Medicare-for-all system of health coverage.

Merkley has been an outspoken opponent of the Trump administration’s hard-line immigration and border security policies. He traveled to the border with Mexico throughout 2018, visiting detention facilities and reporting on his findings.

The senator was an early opponent of migrant family separations at the border in 2018, setting off a national debate when he tried to visit a holding facility and was turned away — a trip he documented live on social media. Soon after, other lawmakers started traveling to the border. Later this month, he plans to visit the border area once again.

Merkley was also active in opposing President Trump’s first Supreme Court picks. In 2017, he staged a protest of now-Justice Neil M. Gorsuch, which attracted widespread attention. He took to the Senate floor for about 15 straight hours to speak out against Gorsuch’s nomination, remaining in the chamber overnight to deliver his remarks.

Merkley said the decision on the presidential race was “the most difficult decision I’ve addressed in my life.”