WASHINGTON—Longtime Representative Edward Markey was sworn in as junior senator from Massachusetts Tuesday, joining his new colleagues just as the Senate was attempting to resolve its latest partisan crisis.

Vice President Joe Biden delivered the oath to Markey, who was flanked by Senator Elizabeth Warren and the William “Mo’’ Cowan, who held the Senate seat temporarily after it was vacated by Secretary of State John F. Kerry. Markey won the seat in a special election on June 25, defeating Republican Gabriel Gomez.

Markey, 67, was a veteran of the House, but is now a rookie in the Senate.

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Markey’s swearing-in ceremony was briefly postponed while Senate majority leader Harry Reid announced that an agreement had been reached to approve one President Obama’s nominees, Richard Cordray, as head of the Consumer Financial Protection Bureau. It was the first step in a tentative deal to end a stalemate over seven White House nominees.

Also on hand for Markey’s swearing in were Governor Deval Patrick of Massachusetts, and House Democrats Nancy Pelosi, the minority leader, and Representative James McGovern of Worcester.

Reid had spent the last week threatening to change Senate filibuster rules and use the “nuclear option” to push nominees through without Republican support. Reid’s threat marked a high level of brinksmanship in the Senate, which Markey said he joined because he felt stifled as a member of the minority party in the House.

The irony was not lost on the new senator.

“My decision was to come to the Senate in order to be in the majority and to make government work for every family in our country,’’ Markey said in an interview before his swearing-in ceremony. “On the day that I arrive, we are having a debate over whether or not this institution can work for every family.”

Markey said he had been ready to support Reid’s efforts to change the filibuster rules and ram through the nominees on a simple majority vote. “I intend on casting an ‘aye’ vote to ensure that the Republican obstructionists cannot paralyze the Senate as the Republicans have paralyzed the House of Representatives,” Markey said.

The Malden Democrat was the ranking member of the House Natural Resources Committee and a leading advocate for policies to fight climate change. On Tuesday, he became the Senate’s most junior member — more junior than Warren, who was sworn in in January as part of the 2013 freshman class.

Markey said he has no regrets in taking the cut in seniority, due largely to his frustration with the House, which he said has become “dysfunctional.”

Though he knows the culture is different between the 435 member House and the 100 member Senate, he said he knows the rules and the issues and plans on “playing a role this year.”

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