U.S. Sen. Jeff Merkley has another title — ambassador — to burnish his resume, and he has President Donald Trump to thank for it.

Trump on Thursday quietly nominated Merkley to be a U.S. representative to the United Nations General Assembly. The designation comes with the title of ambassador.

Why would Trump bestow such a high-profile assignment on an outspoken critic who's a favorite of the progressive left and potential opponent in 2020?

Turns out even hyper-partisan Washington D.C., has some room for intra-party cooperation. Merkley needed the title to attend the U.N. General Assembly meeting next week. He was invited to attend, as senior Washington pols often are — because of his seat on the Senate Foreign Relations Committee and because of his activist stance on immigration issues.

"It's a pro-forma thing," said Merkley staffer Ray Zaccaro. "This is a credential he needs to participate."

It's a temporary designation. Merkley will not be replacing the permanent U.N. Ambassador Nikki Haley.

Trump also nominated three other temporary U.N. ambassadors – all of them closer to the President on the ideological spectrum -- U.S. Sen. Ron Johnson, R-Wisconsin; Margarita Palau-Hernandez, a California lawyer and Republican fundraiser; and Elizabeth Erin Walsh, a former Trump staffer.

Merkley has made no secret of his presidential ambitions. He is headed to key primary state New Hampshire again this weekend where he will headline two Democratic fundraisers.

Oregon's junior senator has raised his profile, in part, by playing Trump's arch-nemesis. Merkley enjoyed what The New York Times called a "breakout moment" this summer when he played a lead role in exposing the administration's immigration policies that separated children from their parents and held them in detention centers.

Detaining immigrant children didn't start with the Trump era. But the number has soared to more than 12,000.

Merkley again went after Trump this week when he pointed out his administration had transferred $10 million from FEMA to Immigration and Customs Enforcement just as Hurricane Florence threatened to wreak havoc in the Carolinas. The money was earmarked to provide more beds for migrant detainees.

—Jeff Manning