Lynch said he and Capuano sat down over cappuccino in Boston's North End neighborhood on Saturday, which Capuano once represented but is now in Lynch's district, to talk it over. At that point, Capuano hadn't yet made his decision, Lynch said.

But he decided by Tuesday, announcing to the public he would not be pursuing the seat that Sen. John KerryJohn Forbes KerryOvernight Defense: Pompeo clears Senate panel, on track for confirmation | Retired officers oppose Haspel for CIA director | Iran, Syria on agenda for Macron visit Senate must save itself by confirming Mike Pompeo Pompeo faces pivotal vote MORE (D-Mass.) will vacate when he is confirmed as secretary of State, which appears likely.

Markey has already announced his intention to run, and shortly after jumping into the race received the backing of a number of prominent Democrats, notably Kerry himself and the Democratic Senatorial Campaign Committee (DSCC).

With the backing of the DSCC and his solid fundraising ability, Markey would be a formidable opponent to any other Democrat hoping to jump into the race, but Capuano's decision not to run further complicates matters for Lynch.

Some Democrats believe he'd have a tougher time winning against just Markey, without Capuano to further split the vote.

Lynch acknowledged his run now "would be uphill," but said Capuano's choice not to run would not ultimately make or break his own decision.

"It will be an issue; I just don't know if it determines the outcome of the race," he said of a potential two-person primary field.

He said he's still going around Massachusetts looking for support for his bid, and admitted that he had been pursuing union supporters, and "might" have some of their backing.

Lynch has previously said he hopes to announce his plans after Kerry's confirmation process is completed, but with the timeline looking to draw out over the next month, he said he might make it official then.

"If we come to a decision before [Kerry is confirmed], we're probably going to have to just go on out," he said.