Speaker Nancy Pelosi (D-Calif.) on Thursday defended her decision to endorse Rep. Joe Kennedy III over Sen. Ed Markey in the Massachusetts Senate Democratic primary, citing her close ties to the Kennedy clan — and her "loyalty" to members of the House caucus.

"I'm serving with him in the House, and I'm a big supporter of people in the House," Pelosi said in a virtual interview with The Washington Post. "You can ask my colleagues. My loyalty is to them."

Pelosi's endorsement of Kennedy, announced just hours earlier, came as a surprise and quickly made waves on Capitol Hill. Not only does Pelosi typically remain neutral — at least publicly — in internal party battles, but she also has a long history of supporting incumbents over challengers regardless of other considerations, even political ideology.

That blanket support for sitting lawmakers has been adopted also by the party's campaign arm — the Democratic Congressional Campaign Committee (DCCC) — which implemented a controversial new policy this election cycle barring campaign contracts to political firms working for primary challengers.

Party leaders defended that decision, citing the need to maintain control of the House under an adversarial president. But progressives on and off of Capitol Hill howled at the change, arguing it was designed merely to insulate well-heeled establishment lawmakers from lesser-known challengers — and new ideas.

The issue has become a contentious one in recent years with the arrival of a new crop of liberal firebrands, some of whom have bucked the party's unwritten rules by endorsing primary candidates challenging their own colleagues. And some of those progressive lawmakers moved quickly Thursday to highlight Pelosi's decision to enter the Markey-Kennedy race as proof that their own interventions were justified all along.

"No one gets to complain about primary challenges again,” tweeted Rep. Alexandria Ocasio-Cortez (D-N.Y.), who defeated veteran Rep. Joseph Crowley (D-N.Y.) in a stunning primary upset in 2018. “So @dccc, when can we expect you to reverse your blacklist policy against primary orgs?”

Pelosi acknowledged that she doesn't normally jump into primary contests and said she wouldn't have done so in Massachusetts if there was any chance a Republican could pick up the Senate seat.

"I would probably not be getting engaged in a primary in an election where it could impact whether a Democrat or a Republican could win," she told the Post. "But this will be a Democratic seat, and I feel at peace with the decision."

The Speaker also attributed her decision, in part, to Kennedy's work stumping on the campaign trail in 2018, when Democrats seized the House and returned the gavel to Pelosi after eight years in the minority wilderness.

"That's why so many of our freshmen members are endorsing him as well," she said.

And much of the decision, Pelosi said, was based on the decades-long connection between the Kennedy family and her own. She noted that her father, as mayor of Baltimore, had played a central role in Maryland to elect President John F. Kennedy. And her tenure in the House overlapped with that of Kennedy's father, Joseph Kennedy II (D-Mass.), and his second cousin, Patrick Kennedy (D-R.I.).

"I wasn't too happy with some of the assault that I saw made on the Kennedy family," she said. "And I thought: Joe didn't ask me to do endorse him, but I felt [it] imperative to do so."

Yet Pelosi had also worked for decades with Markey, a liberal firebrand in his own right who was tapped by Pelosi to lead a key House climate committee in 2007 and has since joined Ocasio-Cortez in championing the Green New Deal.

Markey's progressive supporters are up in arms that the same Democratic leader who'd discouraged primary endorsements against establishment incumbents is now siding against a liberal one.

“Pelosi endorsing Kennedy is just evidence that supports @EdMarkey case against dynasty and smoke filled rooms anointing candidates," tweeted Rep. Ro Khanna (D-Calif.). "Bold prediction: it will backfire."

The Massachusetts primary is scheduled for Sept. 1.