Anticipating highly competitive Congressional elections, advocacy groups are pivoting swiftly into campaign mode as they seek to preserve or overturn legislation they have intensely lobbied for the past few years.

Liberal and labor groups, which have backed Democratic measures such as health care and financial regulatory reform, are endeavoring to prevent big Republican gains they fear could threaten their favored initiatives.

Meanwhile, conservative and business organizations such as the U.S. Chamber of Commerce are working to elect a Republican majority they hope will undo or at least amend these measures.

Targeting vulnerable Senate and House seats, groups on both sides are expected to pour tens of millions of dollars into loosely regulated independent election expenditures.

Health Care for America Now, a group funded by unions and other progressive organizations, spent $47 million over the past two years nudging Congress to approve the massive health care legislation.

While HCAN retrenched after President Barack Obama signed the health care bill into law in March, the nonprofit group is ramping up this summer as lawmakers campaign for re-election.

“Like everyone else, our activities will increase in August and everything shifts into overdrive by Labor Day,” said Ethan Rome, who took over as HCAN’s executive director in April.

Rome said his group will be paying particular attention to 25 to 30 districts represented by Democratic lawmakers, in states such as Colorado and Ohio, who may need some bucking up because of their votes for the health care plan.

Such support is intended to counter the pounding these lawmakers are expected to receive from other groups including the U.S. Chamber of Commerce, which wants to replace those Democrats with Republicans who are pledging to repeal the health care law and who oppose the Wall Street reform effort.

Bruce Josten, the chamber’s executive vice president for government affairs, said his group would be supporting candidates “who don’t subscribe to the health care law.”

Josten said the chamber hopes to raise as much as $75 million to spend on election activities in states such as Pennsylvania, Ohio and Illinois where there are both critical Senate and House seats in play.

He said chamber activities will include ads, phone banks, mailings and the deployment of 400 to 600 volunteers to help on the ground.

So far this year the chamber has spent $1.4 million in independent expenditures, including more than $1 million on Republican Scott Brown’s upset in the Massachusetts Senate race.

Democrats fear the Supreme Court’s Citizens United decision, which paved the way for more corporate and union political expenditures, will result in a spending spree by businesses and their coalitions this fall.

But Josten said Congressional efforts to require greater disclosure of corporate giving has given pause to companies that often do not want to be identified with partisan campaigns. He noted that the House-approved DISCLOSE Act, which the chamber opposes, would require ads run by the chamber or other groups to publicize their top donors.

“That does not compel companies to write checks,” he said. “It is clearly a factor in the decision-making of corporations if in fact it does become law.”

It is unclear whether the DISCLOSE Act will become law this year as the Senate has not yet taken it up.

Even though the Citizens United decision also could benefit unions and liberal groups such as HCAN, those activists respond that they always will be outspent by big corporations, which often anonymously underwrite partisan efforts spearheaded by outside groups.

Democrats have been circulating a memo that claims conservative groups, such as Americans for Job Security and American Crossroads, led by former Bush political adviser Karl Rove, are likely to spend at least $300 million this election cycle.

Jon Youngdahl, the national political director for the Service Employees International Union, said the anticipated increase in corporate political spending “means we are going to have to be smarter with our messaging.”

The quandary for labor, Youngdahl said, is that many of its members, while supportive of Obama, are frustrated by a lack of progress on their agenda in Washington, D.C., and the sluggish economy.

“Frustration isn’t a great motivation to vote,” he noted.

Youngdahl added that the unions are hoping to turn the election into a referendum on the excesses of Wall Street and corporate America in general. While the SEIU expended considerable resources promoting the Democrats’ health care plan, Youngdahl said the union will use the issue selectively in the election since many provisions have not yet gone into effect.

He said the SEIU has budgeted $44 million — which includes targeting 15 House and four Senate races — for this year’s elections.

MoveOn.org, another well-funded liberal player, will be encouraging its members to flock to lawmakers’ town hall meetings in August, much as conservatives did last year to protest health care legislation.

MoveOn members want to stiffen the resolve of some Democrats, who they believe compromised too much on health care by dropping the public insurance option.

Ilyse Hogue, director of political advocacy and communications for MoveOn, said that while the group worked hard to put Democrats in the majority, one lesson it learned from the legislative wrangling is “not all Democrats are created equal.”

Indeed, a number of liberal groups have taken sides in Democratic primaries this year campaigning against some lawmakers, such as Sen. Blanche Lincoln (D-Ark.), who they felt were too conservative. Democratic lawmakers who did not support the health care bill are not expected to get much election help from the unions and progressive groups.

Nevertheless, Hogue said that MoveOn will be active in the fall elections helping many Democrats in part because it believes a Republican majority may be a worse alternative. She said the goal is to ensure “we don’t have Speaker Boehner come fall,” referring to House Minority Leader John Boehner (R-Ohio).

On the other side of the political spectrum, FreedomWorks, the conservative group led by former Majority Leader Dick Armey (R-Texas), in the past year staged a number of raucous rallies with tea party activists outside the Capitol protesting the health care legislation.

Now the group has enthusiastically plunged into the campaign season, supplementing the ground efforts in 60 House and 15 Senate races, according to Rob Jordan, director of the FreedomWorks Political Action Committee.

“For our members, they had had it after health care passed,” he said. “They are ready to throw the bums out.”