After months on the sidelines, major liberal donors including the financier George Soros are preparing to inject up to $100 million into independent groups to aid Democrats’ chances this fall. But instead of going head to head with the conservative super PACs and outside groups that have flooded the presidential and congressional campaigns with negative advertising, the donors are focusing on grass-roots organizing, voter registration and Democratic turnout.

The departure from the conservatives’ approach, which helped Republicans wrest control of the House in 2010, partly reflects liberal donors’ objections to the Supreme Court’s Citizens United decision, which paved the way for super PACs and unbridled campaign spending.

But in interviews, donors and strategists involved in the effort said they also did not believe they could match advertising spending by leading conservative groups like American Crossroads and Americans for Prosperity, and instead wanted to exploit what they see as the Democrats’ advantage in grass-roots organizing.

“Super PACs are critically important,” said Rob Stein, the founder of the Democracy Alliance, a group of liberal donors who will convene near Miami this week to discuss where to steer their money this year. But the liberal groups, he said, believe that local efforts and outreach through social media “can have an enormous impact in battleground states in 2012.”

In a move likely to draw in other major donors, Soros will contribute $1 million each to America Votes, a group that coordinates political activity for left-leaning environmental, abortion rights and civil rights groups, and American Bridge 21st Century, a super PAC that focuses on election-oriented research. The donations will be Soros’ first major contributions of the 2012 election cycle.

“George Soros believes the Supreme Court’s decision in Citizens United opened the floodgates to special interests’ paying for political ads,” said Michael Vachon, a spokesman for Soros. “There is no way those concerned with the public interest can compete with them. Soros has always focused his political giving on grass-roots organizing and holding conservatives accountable for the flawed policies they promote. His support of these groups is consistent with those views.”

On Monday, in an indication that he does not expect significant advertising spending from Democratic-leaning outside groups at this stage, President Barack Obama unveiled a $25 million ad campaign against Mitt Romney, the presumptive Republican nominee.

A super PAC founded by two former Obama aides, Priorities USA Action, has struggled to raise money against better-financed conservative groups like American Crossroads, which expects to spend $300 million on the presidential, House and Senate elections.

Those difficulties stem in part from Obama’s past opposition to spending by outside groups, which has dampened donor enthusiasm despite his about-face this year. But it also reflects how major liberal donors and independent groups have focused since 2004 on creating a permanent infrastructure of liberal research and voter-outreach groups. That year, liberal groups spent more than $200 million on advertising and grass-roots activity in a failed bid to deprive President George W. Bush of a second term.

Conservative independent groups, including super PACs that can raise and spend unlimited amounts of money on election ads, dominated the advertising wars in 2010, helping Republicans make major gains in Congress, and their money has had a similar impact so far in this cycle.

“The idea that we’re going to engage in an arms race on advertising with the Republicans is not appealing to many liberal donors,” said David Brock, the founder of American Bridge 21st Century.

The advertising-oriented Democratic super PACs, including Priorities USA and two groups founded to back Democrats in Congress, remain on the list of organizations that the Democracy Alliance recommends to its members. Robert McKay, who is the chairman of the Democracy Alliance and sits on the board of Priorities USA, said the $100 million expected to be spent this year by alliance members would include some money for election ads, but would most likely favor grass-roots organizing and research groups.

“There is a bias towards funding infrastructure as it relates to the elections,” McKay said. “That means get-out-the-vote efforts” directed toward young voters, single women, black voters and Latinos, he said.

Organizations likely to be a part of the effort include Catalist, which creates voter lists for allied liberal groups; ProgressNow, a network of state-based websites for liberal opinion and activism; and the Latino Engagement Fund, a new group that works to register and turn out Latino voters for Democrats. Conservative independent groups are financing similar outreach to Latino voters: the American Action Network, which spent $26 million against Democratic candidates in 2010, last year unveiled the Hispanic Leadership Network, which will seek to mobilize center-right Latino voters.

Liberals outside the Democracy Alliance are also likely to make significant contributions, as are labor unions, which plan to spend up to $400 million on state, local and federal races, and advocacy groups like the Sierra Club.

Some groups will pay for both advertising and organizing. PAC(PLUS), a super PAC founded by the San Francisco philanthropist Steve Phillips, a member of the Democracy Alliance, expects to spend about $10 million on Latino voters in six states, with a heavy emphasis on Arizona, which the Obama campaign is seeking to turn into a battleground state. Half of PAC(PLUS) spending will go to enrollment and half to advertising.

“You can dump 10 or 20 million in TV ads in Ohio and try to reach the persuadable swing voters there, or you can up voter turnout among Latinos in Colorado and Arizona and win that way,” Phillips said. “It’s much cheaper.”

© 2012 The New York Times Company