American Crossroads, the biggest of the Republican “super PACs,” is planning to begin its first major anti-Obama advertising blitz of the year, a moment the Obama re-election campaign has been girding for and another sign that the general election is starting in earnest.

With an anticipated bank account of more than $200 million, officials at American Crossroads said they would probably begin their campaign this month. But they said they would focus the bulk of the first phase from May through July, which they believe is a critical period for making an impression on voters, before summer vacations and the party conventions take place.

Steven J. Law, the group’s leader, said the ads would address the challenge of unseating a president who polls show is viewed favorably even though many people disapprove of his handling of the economy. Basically, Mr. Law said, “how to dislodge voters from him.”

The ultimate goal of the Crossroads campaign, Mr. Law said, would be to better connect Americans’ disappointment with the economy to their views of the president, especially among crucial swing voters.