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.”