With just more than six months to go before the November elections, two distinct strategies have emerged among political interest groups: an air war on the right and a ground game on the left.

A cadre of super PACs and nonprofit groups backing Republicans plans to spend more than $450 million to oppose President Obama and other Democrats, relying almost exclusively on waves of radio and television ads that will wash over battleground states in coming months. The onslaught has begun as Republican groups strive to damage Obama’s standing ahead of the parties’ national conventions this summer.

Liberal groups, by contrast, are focused more heavily on grass-roots organizing, led by labor unions that hope to spend more than $400 million to rally their members and nonunionized voters against likely GOP presidential nominee Mitt Romney and other Republicans.

The differing strategies mean that voters, particularly in swing states, probably will be inundated with television advertisements attacking Obama well beyond whatever the Romney campaign airs. At the same time, many voters also will encounter swarms of canvassers handing out fliers and knocking on doors in support of Democrats.

Each side is banking on the idea that its approach will help shift the balance in what is shaping up to be a close-fought campaign.

The efforts underscore the prominent role that interest groups will play in the 2012 elections, when their spending could exceed $1 billion. For Democrats, unions are particularly crucial because liberal super PACs and nonprofit organizations have fallen far short of their conservative counterparts in fundraising.

The spending by interest groups comes on top of ambitious goals set by the Obama and Romney campaigns, each of which has suggested to donors that it could meet or exceed Obama’s record-setting haul of $745 million in 2008.

Groups on both sides also are taking full advantage of court rulings that have loosened limits on election activities. Well-funded conservative groups are attracting $10 million checks from wealthy financiers and corporations that are no longer restricted in their political giving. Labor organizations, meanwhile, have been able to cast away regulations that made it harder for them to fish for votes among nonunion households.

The contrast already is becoming clear in swing states such as Ohio, where the conservative group Crossroads GPS is blitzing the airwaves this month with TV ads attacking Obama’s energy policies. Labor-backed groups in the state, in the meantime, are focused on canvassing drives to register voters, support Democrats and push for the defeat of a proposed anti-union state ballot initiative.

“What we’re doing is much more grass-roots than what the other side is doing,” said Michael Gillis, a spokesman for the AFL-CIO of Ohio, which has about 600,000 members. “We have Ohio working people talking to Ohio working people about the issues. It’s a sharp contrast with the air war that the other side wages with their rhetoric and outside money financing the whole thing.”

But the major conservative groups believe grass-roots organizing is best left to Romney and the Republican National Committee.

“A lot of us don’t think it’s efficient for outside groups to do ground-game activities,” said one super PAC official, who spoke on condition of anonymity to discuss strategy. “The campaign finance laws are set up to allow the parties to do that, and we believe they do it quite well. Our added value will be on the airwaves.”

Labor organizations have long been shackled by limits on their political activity that required them to confine much of their canvassing and get-out-the-vote effort to union members, whose numbers have been steadily dwindling.

But now, unions have concluded they can ignore such restrictions under the 2010 Supreme Court ruling in Citizens United v. Federal Election Commission , which freed corporations and unions to spend unlimited funds on elections.

The ruling paved the way for the super PACs that have had enormous success, particularly among conservatives, in raising money from wealthy donors with no restrictions. But labor lawyers say the decision also means that unions can use their treasury funds to encourage nonunion members to vote for specific candidates, a tactic that was barred under previous laws.

“It’s always been the bread and butter of labor that we had our ground game, but this really expands the possibilities,” said Eddie Vale, spokesman for Workers’ Voice, a super PAC formed by the AFL-CIO that plans to focus on grass-roots organizing.

Democrats are relying on such efforts to help counter the high-dollar fundraising of conservative groups. Three of the leading organizations backing Romney and other Republicans — American Crossroads, Restore Our Future and the American Action Network — plan to spend well over $300 million between now and November. Tens of millions more will be spent by Republican-leaning interest groups such as the U.S. Chamber of Commerce.

Jonathan Collegio, a spokesman for the American Crossroads and Crossroads GPS groups, said independent conservative groups are “a counterbalance to the long-term influence that labor unions have had on the political process for decades. . . . What’s going on on the right is clearly balanced out by the labor unions.”

The environmental lobby is also a major player on the left. The League of Conservation Voters, for example, is planning a mix of grass-roots organizing and media buys, including a pro-Obama ad campaign to be launched next week.

Union leaders are coy about their specific plans but say they hope to match the estimated $450 million spent by unions and their political action committees in 2008.

The biggest spender, the American Federation of State, County and Municipal Employees, says it expects to surpass the $93 million it spent four years ago on federal, state and local elections. Another major player, the National Education Association, a teachers union, also hopes to beat its 2008 total of $50 million, officials said.

“There are so many different ways to go at voters, but at some point what’s really going to matter is who’s in your network, who’s in your community, and who do you trust?” said Carrie Pugh, campaign manager for the 3.2 million-member NEA. “We are banking on our members.”