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Sen. Tom Udall (D-N.M.) announced last week he would not seek re-election in New Mexico's upcoming 2020 campaign, opening up a key seat for the party in what could be a competitive election, per The New York Times. On Monday, Rep. Ben Ray Luján (D-N.M.) formally declared on Twitter that he will contend for Udall's vacant seat.

The Hill reports that he is already the frontrunner to take Udall's spot. But the decision does come at an interesting time for the 46-year-old Luján, the fourth highest ranking Democrat in the House who was expected to continue to rise. In November, Luján, whom Politico described as a close ally as House Speaker Nancy Pelosi (D-Calif.), was elected as Assistant Speaker, a role he has occupied since the new year began.

At the time, The Hill reported that Pelosi was seeking a way to give Luján a "leg up" to succeed her as future Speaker. But KOAT News political expert Brian Sanderoff said that Luján's Senate campaign means he is expected to give up his seat in the House.

For his part, Luján said in his announcement video that while he was proud to represent New Mexico in the house, "to move forward, we've got to fix the Senate." And, after all, as Sanderoff told KOAT, it's rare when a Senate seat opens up. "When one comes up, it's pretty special," he said.