Mr. Kerrey, a former Nebraska governor and senator who left office a dozen years ago to run the New School in Manhattan, has responded with nearly half a million dollars in commercials of his own. That is a considerable expense for his fledgling campaign, and almost as much as the amount of cash he had on hand at the end of March.

The conservative groups’ strategy makes clear that they — traditional Republican allies like the Chamber of Commerce along with newer players like the Karl Rove-backed Crossroads GPS and Americans for Prosperity, a Koch brothers-affiliated venture, among others — intend to be even more aggressive this year than they were in 2010, when they greatly expanded the role of outside money in Congressional elections.

By forcing their opponents to respond with expensive ad campaigns of their own, these conservative groups are achieving at least part of their goal. But their strategy is not without its risks, namely that they are dumping money into advertising long before voters are really paying attention.

The focus on the Senate reflects conservative hopes for a sweep of the White House and both branches of Congress, and frustration with the Senate’s role in blocking legislation passed by the Republican-controlled House, especially on the budget and fiscal issues that have come to dominate the debate in Washington.

“You could argue that what happens in the Senate and the House matters more on a day-to-day basis, on key legislative matters, than an administration does,” said Tim Phillips, president of Americans for Prosperity.

The Chamber of Commerce, which, according to the Center for Responsive Politics, spent more than $30 million on Congressional races in 2010, said it will commit “measurably more” this year to House and Senate campaigns in what officials called the most significant political effort in its 100-year history.

“We’re going to shape the environment now instead of waiting for the environment that comes to us later,” said Rob Engstrom, national political director for the Chamber of Commerce. For over a year, the group has enlisted strategists to comb through candidates’ voting records and public statements, looking for material useful in an ad.