Rep. Edward J. Markey plans to run for the Senate seat that is likely to be vacated by Sen. John F. Kerry, his fellow Massachusetts Democrat and President Obama’s nominee for secretary of state.

Markey, 66, has served in the House since 1976. The dean of the Massachusetts delegation said in a statement annnouncing his decision, “There is so much at stake.”

“I refuse to allow the Tea Party-dominated Republican Party to lead us off the fiscal cliff and into recession,” Markey said. “I won’t allow the NRA to obstruct an assault weapons ban yet again. I will not sit back and allow oil and coal industry lobbyists to thwart our clean energy future or extremists to restrict women’s rights and health care.”

Kerry is expected to be easily confirmed, and the vacant seat will be filled by a special election next year. Gov. Deval Patrick (D) will appoint an interim senator until the special election winner takes office. In 2009, when Sen. Edward M. Kennedy (D) died, the governor chose Paul Kirk, who served as a placeholder and did not run in the special election.

Markey is the first major candidate to announce. Other Democrats mentioned as possible candidates are Reps. Michael Capuano and Stephen Lynch, former congressman Martin Meehan and state Attorney General Martha Coakley.

Scott Brown, who lost his Senate seat to Democrat Elizabeth Warren in November, is the overwhelming Republican favorite.