The long list of George Miller’s prominent official titles being unfurled is a reminder of why he is easily the most important member of the current Congress who has announced a retirement.

But his informal position — at the very center of  Nancy Pelosi’s inner circle — makes Monday’s news of his planned departure especially consequential.

Miller has been her uniquely influential patron, confidant, consigliere, travel buddy and liberal soul mate during the past three decades. More than any other lawmaker, he made and has maintained his fellow Californian’s hold on power in the House Democratic Caucus.

Even the most loyal soldiers in the party have already given up all but theoretical hope of winning back the House this November. And so Miller’s leave-taking will inevitably be interpreted as something more significant than that: a sign that Pelosi will soon be contemplating (if she isn’t already) bringing a voluntary end to her 12-year run as party leader once the midterm elections are over.

Maybe Miller acted all on his own, concluding that his children were right when they said 40 years in office had a nice ring to it. But, given their relationship, there’s also reason to suspect he would be among the first to be clued-in if his influential services as Pelosi’s sidekick weren’t going to be required in the 114th Congress.

At best, with Miller gone it will be considerably lonelier at the top for her, at least in the short term. At worst, things will be considerably more dangerous if she wants to remain in leadership.

When Pelosi decided to make her big up-or-out move to crack the glass ceiling in her party’s leadership in 1999, starting a three-year quest to become whip, Miller was among the first of her colleagues urging her on and rounding up support for her ultimately successful campaign.

When Pelosi presided over House passage of the health care law in 2010, Miller was rushing off the floor to envelop the speaker’s husband, Paul Pelosi, in an embrace.

And after the Democrats lost the House later that year, Miller persuaded a crucial bloc of disgruntled centrists to back down from their coup attempt, arguing that the fault belonged much more with the Obama administration than with their own leader.

Since Pennsylvania’s John P. Murtha died four years ago, the depth chart for Pelosi’s kitchen cabinet has been marked by a big gap. Nobody is obviously positioned to take the lead in shielding her from rivals, rallying support behind her and otherwise playing the heavy the way the unapologetically combative and physically imposing Miller did.

Among the lawmakers who could fill at least part of Miller’s space are New Jersey’s Robert E. Andrews and Connecticut’s Rosa DeLauro. Their status in Pelosi’s inner sanctum was cemented a year ago, when she picked them to be co-heads of the Democratic Steering and Policy Committee, which Miller had led for the previous decade at Pelosi’s behest. The panel is the leadership’s agent not only in formulating legislative policy, but also in making carrot-or-stick committee assignments.

In addition to that insider’s power base, Miller has been chairman or ranking Democrat on the House Education and the Workforce Committee for the past 13 years, and for a decade before that he held the party’s top seat on Natural Resources, including four years as chairman. He founded the Congressional Progressive Caucus in 1991 and also chaired a select House committee on children and family issues.

In all those positions he was an unabashed progressive, reliably advocating more public education funding, stronger workers’ rights, and tougher consumer safety and environmental laws. He was instrumental in assembling the Democratic votes necessary to enact the health care law and also in crafting the bipartisan deal that led to enactment of the 2001 No Child Left Behind education policy overhaul.

He is the fourth most senior member of the House and the dean of its largest delegation, having easily won a district encompassing most of Contra Costa County, north of San Francisco, in 20 consecutive elections. (Obama twice took more than two-thirds of the vote in the district, so it looks guaranteed to stay in Democratic hands; several state legislators on Monday were eager to test the waters.)

Miller, who came to the Hill as a 29-year-old lawyer, and fellow Californian Henry A. Waxman are the only members remaining from the giant class of Democrats elected in 1974, after Watergate forced President Richard M. Nixon’s resignation. Because he will turn 69 this spring, Miller will have been a House member for 58 percent of his life at the time of his departure. And only about 50 people in American history have been members of Congress longer than four decades.

Beyond reviewing much of that history, Miller and Pelosi both issued mutual-admiration-society statements Monday — notable mainly because they both used an identical phrase to hint that he may have already lined up a career after leaving Congress.

“I look forward to one last year in Congress fighting the good fight and then working in new venues on the issues that have inspired me,” said Miller, whose last legislative crusade will be spearheading Obama’s long-shot, wedge-issue campaign to win an increase in the minimum wage.

“My sadness at his departure from Congress in 2015 is mitigated only by my certainty that he will utilize his exemplary knowledge and skills in a new venue,” Pelosi said.