Rep. Stephen Lynch (D-Mass.) will announce his Senate bid in Massachusetts on Thursday, launching a primary challenge to Rep. Edward MarkeyEd MarkeyTrump's steps on Iran show cooperation with Congress is possible FCC votes to advance net neutrality repeal Senate Dems appeal to 'everyone who uses the internet' on net neutrality roll back MORE (D-Mass.) for the seat being vacated by Sen. John KerryJohn KerryJohn Kerry channels Yoda in tweetstorm John Kerry goes on tweetstorm as Senate eyes Iran legislation John Kerry's advice to Harvard grads: Learn Russian MORE (D-Mass.) on Tuesday.

He will make the official announcement surrounded by friends and supporters at the IronWorkers' Hall in Boston at 4 p.m. on Thursday, according to his spokesman, Scott Ferson.

Lynch will spend the morning campaigning, likely at local diners, in Springfield and Worcester, Mass.

The primary for the Senate seat, now open following Kerry's confirmation as secretary of State, will occur on April 30; the general election will occur on June 25.

Markey was the first candidate to announce his intentions to run for the seat, and has, since announcing, racked up numerous big-name endorsements in both national and Massachusetts Democratic politics. Notably, he has the backing of Kerry himself, and the Democratic Senatorial Campaign Committee.

Markey also has millions more cash on hand than Lynch, making this an uphill battle for Lynch by any standards. But he has been preparing his bid for some time, sources say, and hopes that union support could help boost him in the primary.