By Patricia Zengerle
WASHINGTON (Reuters) - A year ago, organized labor was threatening a break with President Barack Obama for being too willing to compromise with Republicans on issues like the debt ceiling, Bush tax cuts, free trade and workplace legislation.
Union leaders said they would no longer walk in lock step with Democratic politicians if they did not support labor's agenda, and hinted at more campaigns like the $10 million they spent in an unsuccessful bid to defeat Democratic Senator Blanche Lincoln in the 2010 Arkansas primary.
But that was last year.
Now, with former executive Mitt Romney likely to win the Republican presidential nomination, tight election races looming in November and attacks on unions by Republicans in state governments, labor is firmly back in the Democratic fold.
There is still some labor discontent - AFL-CIO President Richard Trumka last week said a free trade pact with Colombia was "deeply disappointing" - but unions are strongly supporting Democrats with money and boots on the ground in an election they see as essential to their survival.
"The stakes are very, very high," said Marick Masters, a professor at Michigan's Wayne State University who studies labor and politics.
Unions are deploying hundreds of thousands of members to knock on doors, and are spending big to counter aggressive efforts by Republican Super PAC groups, which have stolen the show so far in fundraising this election cycle.
The emergence of Romney as the probable Republican nominee has also galvanized organized labor. The multi-millionaire former private equity executive has made attacks on "union bosses" a feature of his campaign rhetoric.
"We've seen in the last year nothing but attacks on working people," said Michael Podhorzer, the AFL-CIO's political director.
The American Federation of State, County and Municipal Employees, which has been battling Republican state governments over workers' benefits, plans to spend up to $100 million this year to help Obama and other Democrats.
Other unions will add to that total in donations to campaigns, but also on lobbying, staff and sending members to communicate with voters, register voters and get Democrats to polling places on Election Day.
"If you factor in all the personnel at the state and local level and all the costs of using those personnel, all the PAC money, all the independent expenditures ... you are probably going to see labor spending close to $1 billion on political activity, including lobbying at the local, state and national level in 2012," Masters said.
UNION MONEY TRAILS FAR BEHIND CONSERVATIVES
Union spending will still be overshadowed by outside groups that back Republicans. Just two, American Crossroads and Crossroads GPS, a Super PAC and a conservative non-profit founded with the support of strategist Karl Rove, want to spend up to $300 million on 2012 elections.
Super PACs spent more than $100 million on the Republican primary race, including $41 million from the Romney-allied Restore our Future mostly for negative advertisements attacking rivals Rick Santorum and Newt Gingrich.
To counter the rise of the Republican groups, the AFL-CIO has formed its own Super PAC, Workers' Voices. It has raised just $5.4 million to date and has $4.1 million in cash on hand, but it will use the money for what union's consider their greatest strength, grassroots organization.
The labor federation plans to make 2012 its biggest grassroots organizing effort. Its PAC's money will go to mobilize 400,000 union members during 2012 to get out the vote and knock on doors.
The unions' urgency to help Democrats is a far cry from just a year ago when they grumbled about Obama's free trade agreements with South Korea and Colombia and failure to support "card check" legislation that would have made it easier to unionize work places.
Since then, unions have been enmeshed in major political fights in the Rust Belt against laws that would have stripped them of many of their powers.
In Wisconsin, they gathered signatures to force Republican Governor Scott Walker to face a recall election in June over a new law he championed that curtailed public sector union rights.
Labor was also instrumental in a successful campaign to strike down a law in November backed by Ohio's Republican governor to limit state workers' bargaining rights.
Unions will need their organizing strength, since their donations have come nowhere near the level of just a few wealthy donors who have given millions of dollars to Republican causes.
For example, casino mogul Sheldon Adelson and his family invested $16 million in a Super PAC that backed Gingrich, plus $5 million in another PAC set up to defend the Republican majority in the U.S. House of Representatives.
In contrast, labor unions gave about $1.4 million to three Democratic PACs in the first three months of 2012, according to federal filings on Friday.
(Additional reporting by Alina Selyukh and Alexander Cohen; Editing by Alistair Bell and Eric Walsh)