Rep. George Miller (Calif.) was among the first Democrats to push for greater federal oversight and accountability for the nation’s primary and secondary schools. (Chip Somodevilla/Getty Images)

Rep. George Miller’s decision to retire after 40 years in Congress, coming after the announced retirement of Sen. Tom Harkin (Iowa), means Democrats will lose their two strongest congressional leaders on education issues at the end of the year.

Miller, 68, the ranking member of the House Committee on Education and the Workforce, has led the Democrats on issues surrounding K-12 schools and higher education for more than 15 years.

“He’s been the leading Democratic voice in the House on that whole range of issues — education, working people, low-income and special populations of children,” said Joel Packer, an education lobbyist for the Raben Group who used to work for the National Education Association. “Coupled with Harkin’s retirement, this is going to create a double void, so to speak.”

Miller was elected to Congress in 1974, at age 29, as part of a Democratic wave following the Watergate scandal and is a close friend and confidant of House Minority Leader Nancy Pelosi (D-Calif.). Only four House members have served longer and only one other member of that class remains in the House — Rep. Henry Waxman (D-Calif.).

Miller first informed Pelosi of his decision Wednesday, House Democratic aides said.

Miller lives in Martinez, Calif., and comes from a safe Democratic district, where President Obama won more than two-thirds of the vote in 2008 and 2012. Democrats are heavily favored to retain the seat.

In Washington, he owns and lives in a Capitol Hill group house with Sens. Richard J. Durbin (D-Ill.) and Charles E. Schumer (D-N.Y.) that was the inspiration for the Amazon TV comedy series “Alpha House.”

While a classic liberal, Miller repeatedly broke with the teachers’ unions by promoting ideas such as charter schools, merit pay and a federal requirement that only “highly qualified” teachers be running classrooms.

His differences with the unions, traditionally powerful Democratic allies, sometimes went public. During a 2007 hearing, Miller became irate when Reg Weaver, then president of the National Education Association, abandoned his previous support for merit raises for outstanding teachers and spoke against the idea.

“You can dance around all you want,” Miller said pointedly. “You approved the language.”

Randi Weingarten, president of the American Federation of Teachers, said: “Though we didn’t agree on everything, Mr. Miller was always open to discussing how to best help kids succeed. He held investment in public education to be essential. We are in a critical moment to reclaim the promise of public education. With Mr. Miller’s retirement and that of Senator Tom Harkin, the question remains as to who will walk in these lions’ footsteps, working to ensure that all working people have a fair shot and all children have a great future.”

Miller was among the first Democrats to push for greater federal oversight and accountability for the nation’s primary and secondary schools.

In 2001, he joined with President George W. Bush and now House Speaker John A. Boehner (R-Ohio) as well as the late senator Edward M. Kennedy (D-Mass.) to write No Child Left Behind, which required schools for the first time to publicly report the academic performance of subgroups of students — black, Latino and learning disabled, among them — and to set goals for improvement.

Until that point, schools could camouflage the performance of large swaths of students by reporting only a school’s average test scores, or by declining to report any performance data.

“No one would confuse me and George Miller for ideological soul mates, but during our years serving together on the Education & the Workforce Committee, we got things done on behalf of the American people, thanks in no small part to his dedication and willingness to work for the greater good,” Boehner said in a statement.

No Child Left Behind expired in 2007 but efforts to rewrite the law have stalled amid increasingly bitter debates on Capitol Hill about the appropriate federal role in public education.

Rep. John Kline (R-Minn.), who chairs the education committee, credited Miller for working with Republicans on legislation to encourage the growth of charter schools and to simplify the federal student loan program. Calling Miller a “fierce advocate” for students and workers, he said his “strong leadership and work ethic helped the committee forge bipartisan agreement on a number of critical issues.”

But in recent conversations, Miller indicated that the partisan climate on the Hill was becoming a grind, said Charles Barone, a former top aide to Miller who is now policy director for Democrats for Education Reform, an advocacy group. “He talked about how the climate is now, compared to 10 years ago,” Barone said. “Particularly with the tea partyers; he said they don’t even make eye contact with you. It just makes it less fun.”

With the Republican takeover of the House after the 2010 midterm elections, Miller became the Democrats’ main defensive player against GOP efforts to shrink the federal role in K-12. Last summer, Republicans passed a bill that would cede back to states decisions about how to deal with failing schools, how and whether to evaluate teachers, and how to spend much of the money sent by Washington to educate poor, disabled and non-English-speaking students.

In a raucous, lectern-pounding speech, Miller argued that it would devastate the country’s most vulnerable children.

When Miller was advised that “the gentleman’s time has run out,” he shouted back, “No! You know who’s running out of time? Children are running out of time in this nation,” which sparked a slow clap in the chamber. The bill passed without any Democratic votes.

On another occasion, Miller was on the House floor, arguing after his time had expired, Barone said. “The Republicans were banging the gavel and he just kept talking,” Barone said. “They banged that gavel so loud, the head fell off the gavel.”

At home in his California district, Miller frequently spent time in classrooms, something he said helped him understand the challenges of public education. For a time, he taught a Friday class at an alternative high school for drop-outs who were trying to earn a high school diploma. “They learned from him,” Barone said. “But he learned a lot from them.”

Harkin, who has served in the Senate since 1985, announced a year ago that he would not seek another term. He chairs the Senate Health, Education, Labor and Pensions Committee, which is responsible for rewriting No Child Left Behind. He also chairs the appropriations subcommittee that funds K-12 programs. Senate seniority rules place Sen. Patty Murray (D-Wash.), a onetime preschool teacher, next in line to succeed Harkin.

Rep. Robert E. Andrews (N.J.) is a likely successor to Miller as the ranking Democrat on the education panel. That would not represent a significant change, as Andrews is aligned with Miller on most issues. But when it comes to for-profit colleges, Andrews has been far more supportive than Miller.

Ed O’Keefe and Aaron Blake contributed to this report.