(Image credit: Charles Dharapak/AP Photo)

The nation's largest labor unions, whose support has been a mainstay of the Democratic Party, have set aside their frustrations and disappointments in President Obama and formally lined up behind his bid for a second term.

Today the American Federal of Labor and Congress of Industrial Organizations, or AFL-CIO, the nation's largest labor group,  became the latest to endorse Obama for re-election, pledging "to work with him through the election."

"With our endorsement today, we affirm our faith in him," said AFL-CIO president Richard Trumka in a statement.

At least 12 other major U.S. union groups have already done the same - including the American Federation of State, County and Municipal Employees (AFSCME), the largest single U.S. union with more than 1.6 million members - according to a list provided by the Obama campaign.

Trumka, an Obama ally who also serves on the President's jobs council, had been publicly critical of the administration during last summer's debt ceiling debate, warning Democrats that union members would only support their re-election campaigns "if leaders aren't blocking the wrecking ball and advancing working families' interests."

In October, Trumka warned of political peril for Obama if he's seen as merely "nibbling around the edge" in addressing the nation's high unemployment rate.

"History will judge him and I think working people will judge him," Trumka told reporters at the time.

But today, Trumka sang a different tune, hailing Obama's performance on balance, citing the Recovery Act, a landmark health care overhaul and financial regulatory reform as major accomplishments of Obama's term. He also praised the resurgence of American manufacturing and the U.S. auto industry under Obama's watch.

"Although the labor movement has sometimes differed with the president and often pushed his administration to do more and do it faster, we have never doubted his commitment to working families," Trumka said.

Obama campaign manager Jim Messina said he addressed the union's general board before the endorsement vote, saying via Twitter that both organizations were "working together for an economy built to last. "

While union membership has declined from a decade ago, labor groups remain among the most organized, active and enthusiastic part of the Democratic coalition nationwide and are expected to play a key role in Obama's re-election effort.

"Come election time they can be very significant in terms of mobilizing money and mobilizing people," said Princeton University political historian Julian Zelizer.

"Symbolically, the AFL-CIO endorsement is important because it's a signal that this Democratic base is still on Obama's side, even with all the compromises and disappointments - they're still there," Zelizer said.

Here's a full list of Obama's union endorsements and the dates on which they were announced:

National Education Association (NEA) - July 4, 2011

United Food and Commercial Workers (UFCW) - October 20, 2011

National Air Traffic Controllers Association (NATCA) - November 16, 2011

Retail, Wholesale and Department Store Union (RWDSU) - November 17, 2011

Service Employees International Union (SEIU) - November 19, 2011

American Federation of State, County and Municipal Employees (AFSCME) - December 6, 2011

Communications Workers of America (CWA) - February 2, 2012

American Federation of Teachers (AFT) - February 7, 2012

International Association of Machinists (IAM) - February 24, 2012

United Association of Journeymen and Apprentices of the Plumbing and Pipefitting Industry (UA) - March 1, 2012

United Steel Workers (USW) - March 4, 2012

Laborers International Union of North America (LIUNA) - March 8, 2012

AFL-CIO - March 13, 2012