WASHINGTON — Oregon Sen. Jeff Merkley will not seek the Democratic presidential nomination in 2020, telling NBC News that he believes he can best fight for the issues most important for him by staying in the Senate.

“I’ve never been afraid of a tough battle,” Merkley said in a 45-minute interview Monday evening in his Senate hideaway office, acknowledging the long odds he would have faced in a crowded field. “The question I’ve asked myself is, can I be more effective in making a difference on those being in the 2020 primary or by being in the Senate?"

Merkley, 62, was going to run in 2020 no matter what — either in a crowded Democratic field for president, or for re-election to serve a third Senate term. And as he recently considered his upcoming travel plans, the decision suddenly came into focus.

On the one hand he could make a return trip to Iowa, the leadoff caucus state that is now teeming with fellow U.S. senators eyeing the presidency. Merkley has been to the Hawkeye State nine times since 2016.

Or he could make a visit to another border detention facility holding migrant children — in some cases separated from their parents as they sought asylum and a new life far from violent Central American homes. Since he last visited one last December in Tornillo, Texas, the Trump administration closed what he had called a “tent prison” for children.

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Winning the nomination requires a sustained focus on the early nominating states, and deviating from them for a visit like the ones he’s made to detention centers brings with it the risk of being seen as doing so only for purely electoral reasons.

“There’s an opportunity cost if you’re running,” Merkley said. “It is freeing” not to, he added.

Merkley acknowledged there would have have faced long odds if he ran for president, but said in such a large field anything was possible.

"People who may be the frontrunner today may not be the frontrunner tomorrow,” he said. “If I was coming to this with a vast arsenal of an organization — maybe that would change the calculation some. But it just comes down to that core sense that if we’re going to make things happen, we’ve got to have a Senate that makes things happen.”

Merkley also revealed his decision in a video released Tuesday morning, saying he would continue to work not only for his own re-election but to help Democrats win across the country who are dedicated to tackling what he called three main “mega-challenges” facing the country: the increasing concentration of political power in the hands of the wealthy; the lack of opportunity for many Americans to live a comfortable middle class life; and the existential threat to humanity from a changing climate.

Addressing each will require not only a strong Democrat in the White House, but a Congress — especially a Senate — that is no longer dysfunctional to work with him or her, he said in the interview.