E-mailing a group of Sanders supporters, the Acton Democrat also contemplated the creation of a third, progressive party. But he focused on a reform-from-within approach to push the party to the left.

“There are plenty of conservative Democrats who have been elected, unchallenged, for years if not decades, including at the local and legislative level,” wrote state Senator Jamie Eldridge, a liberal stalwart whom Rosenberg appointed Senate chairman of the financial services committee.

A top ally of Senate President Stanley C. Rosenberg is urging supporters of Senator Bernie Sanders to “take over” the state Democratic Party and unseat sitting legislators, a rare break in State House decorum that deepens the growing rift within the party.


“I personally think the time is ripe … for Sanders supporters/progressives to ‘take over’ the Massachusetts Democratic Party, and have a serious influence on its platform, candidates, and policies,” he wrote.

The friction within the state Democratic Party reflects a national split that emerged during the party’s presidential primary, when Sanders’ populist message tested former secretary of state Hillary Clinton for months, before she ultimately prevailed and secured the nomination. Clinton eked out a win over Sanders in the state’s March primary.

Eldridge said in an interview Wednesday that he did not have specific legislators in mind when he called for primary challenges. He said his comments were aimed at Democrats frustrated that the Legislature is not as liberal as they’d like — despite the state’s left-leaning national reputation.

“If activists are unhappy with how they’re being represented … then, yeah, they should consider running for office,” he said. “I think it’s good in a democracy for there to be more competition.”

Eldridge’s e-mail drew a stern rebuke from state party officials. “Jamie’s divisive rhetoric in calling for a third party, or to ‘take over’ the party we all work hard for, is an insult to every elected Democrat and to the hundreds of activists and volunteers who have worked to promote our shared values,” executive director Jason Cincotti said in a statement.


Eldridge’s push also marks the latest wedge in a deepening schism between the state Senate, which has swung to the left since Rosenberg took the reins last year, and the House, a more deliberate, ideologically conservative chamber.

Last month’s end of the two-year legislative cycle produced unusually acidic recriminations between top House and Senate members — all Democrats. The party has an overwhelming majority in both chambers.

In the days after the session expired, two senators ripped House leaders, including Speaker Robert DeLeo, characterizing them as puppets of big-business groups. Top House members responded that Senate efforts, led by Rosenberg, to assert additional control over the parliamentary process smacked of political naivete.

And Democrats in both chambers raised eyebrows last week when Rosenberg’s communications director Mara Dolan tweeted an unusually pointed critique of other Democrats after a state party committee voted to condemn a charter school expansion. “This just in: Democrats in Massachusetts turn out to be real Democrats after all, vote to oppose increasing charter schools,” she said.

But urging candidates to challenge legislative colleagues in one’s own party is widely considered a serious breach of intraparty politesse. In the past, lawmakers have even protested when Republican governors, such as Mitt Romney, sought to field GOP challengers to incumbent Democrats.

“As a member of House leadership, I would never seek an opponent for a Democratic colleague in either branch,” said state Representative Michael J. Moran, a Brighton Democrat who serves on DeLeo’s leadership team. “It’s just something that isn’t good for the inner workings of the Legislature, long-term.”


Moran, who supported Sanders and was one of several lawmakers to receive the e-mail, added, “I know that Bob DeLeo would never condone or sanction one of his members of leadership seeking an opponent for another Democrat in the Senate or the House.”

Eldridge, who served three terms in the House before winning election to the Senate, ran unsuccessfully for Congress in 2007. He is facing a challenge this fall from Republican Ted Busiek, whom GOP strategists consider an underdog.

Eldridge’s Aug. 8 e-mail — whose recipients included several legislators, State House staffers, and a former state party chairman — was penned in response to a thread in which activists pondered the consequences of either joining or starting a third party. Several respondents indicated that they prefer to reform the Democratic Party from the inside.

In his note, Eldridge wrote of “a hybrid approach,” pointing to the Working Families Party operating in New York and Connecticut, where the WFP runs its own candidates but also backs candidates from other parties who back its agenda.

Eldridge called it “an effective way to move especially Democrats to the left,” citing the party’s role in electing Bill de Blasio mayor of New York in 2013.

Eldridge — whose Senate colleague, Thomas McGee of Lynn, chairs the state party — added, “There is absolutely plenty of room to move Democratic politicians to the left, or run for yourselves.”


Philip W. Johnston, a former state Democratic Party chairman who endorsed Sanders during the primary, said he “love[s] Jamie,” but disagreed with his suggestions.

“I don’t think we need another party,” Johnston said Wednesday. “I think we need to do everything we can to make sure Democrats are acting like Democrats, but I don’t think we should be running against other Democrats.”

“I think that progressives should focus on working within the Democratic Party,” Johnston said. “That’s where the action is. Clearly, the fact that Bernie Sanders attracted 46 percent of the vote during the primary season demonstrates that there’s a split within the Democratic Party, and that means that progressives have a chance to have an influence in the coming years.”

The House-Senate acrimony comes as Democrats, who run the Legislature with vast majorities, are still learning to deal with Governor Charlie Baker, the Republican who ended eight years of Democratic hegemony with his 2014 victory. Over the past two years, Baker has lined up more frequently with DeLeo than Rosenberg on policy issues.

A Rosenberg spokeswoman said Wednesday that he was unavailable for comment.

Jim O’Sullivan can be reached at jim.osullivan@globe.com. Follow him on Twitter at @JOSreports.