Rep. Michael Capuano (D-Mass.) will not seek Sen. John KerryJohn KerrySenators who have felt McCain's wrath talk of their respect for him Dems see huge field emerging to take on Trump Budowsky: Dems need council of war MORE's (D-Mass.) seat in the state's likely special election, a decision that removes one of the few remaining obstacles in Rep. Edward MarkeyEd MarkeyOPINION | Shailene Woodley: US should run on renewable energy by 2050 Dems urge 'transparent and inclusive' nuke policy review Senate confirms former Boeing VP as deputy Defense secretary MORE's (D-Mass.) path to the Democratic nomination.

“After careful consideration, I have decided not to enter the race for U.S. Senate. Instead, I look forward to focusing on the important issues facing the new Congress. My current work in the House and whatever opportunities the future may hold afford me the greatest honor of my life, fighting for the citizens of the commonwealth," he said in a statement.

Capuano's move leaves a question mark over the head of only one other probable contender for the seat: Rep. Stephen Lynch (D-Mass.). He, Capuano and Markey, along with state Sen. Ben Downing (D), were all considered potential contenders for Kerry's seat if, as is expected, the senator is confirmed as secretary of State.

Markey is thus far the only contender to announce his bid, and Downing said last week that he will not pursue the seat. Lynch has said previously that he's preparing for the special, though he hasn't yet made an official decision on jumping into the race.

Markey has received the backing of a number of prominent Massachusetts and national Democrats: notably, Kerry himself; former Rep. Barney Frank (D-Mass.), who has asked to be appointed to the seat as interim senator; and Victoria Kennedy, the widow of former Sen. Edward Kennedy (D-Mass.). 

The Democratic Senatorial Campaign Committee (DSCC) also endorsed him, a move that indicated Democrats were looking to clear the field and avoid a potentially bruising Democratic primary.

It's unclear whether Capuano's decision will affect Lynch's plans. His office did not respond to a request for comment.

But a two-way primary might be more difficult for Lynch to win, as he'd have to go up against the considerable fundraising force of not only Markey himself, but the DSCC. Lynch is considered by many Massachusetts Democrats to be too conservative for the blue state. He voted against President Obama’s healthcare reform bill and opposes abortion rights.

Without Capuano further splitting the vote, Lynch might have a tougher shot at nabbing a majority of Democratic voters in the state.

However, Markey is far from the obvious favorite. 

Boston Democratic Mayor Thomas Menino, who endorsed Sen. Elizabeth WarrenElizabeth WarrenTrump's new communications chief once called him a 'hack' OPINION | Hey Dems, Russia won't define 2018, so why not fix your party's problems instead? The private alternative to the National Flood Insurance Program  MORE (D) in her bid, declined to comment on the race through a spokeswoman, and prominent union leader Mike Monahan told the Boston Herald that Markey was a "weak candidate.”

Monahan’s comments could signal trouble ahead for Markey. Union groups played a large role in get-out-the-vote efforts in 2012, and they’d likely be essential to any Democratic bid this time around. Lynch has union ties from his early work as an iron worker and as a union advocate, and has reportedly been working to rally union support for a bid.

Monahan’s comments reflect the opinion of some in Massachusetts that because Markey hasn’t had a contested primary in over three decades, he may be ill-suited to protect himself against attacks from the probable Republican nominee, former Sen. Scott Brown. 

Some Democrats believe it would be best for Markey to hash out some of his biggest weaknesses — including questions over whether he still resides in Massachusetts, despite spending most of his time in Washington and Maryland — in friendly territory, before facing Brown.

Now all eyes will shift to Lynch. Lynch has said he wants to make his decision after Kerry has gone through the confirmation process. But in light of Capuano’s decision, it might come sooner. The special election will take place within 160 days following Kerry’s resignation, making it likely to occur sometime this summer.

— Updated at 4 p.m.