The TV buy is the first of a large primetime block by a presidential candidate since H. Ross Perot, who ran on the reform party line in 1992. Larry Sabato, executive director of the Center of Politics at the University of Virginia, said the practice was more common in the early days of television.
The Democratic candidate has more cash than his Republican rival, John McCain, because he is not receiving public campaign funds and is permitted to raise virtually unlimited campaign contributions. McCain must keep to spending limits set by the public financing law.
An industry analyst estimated the cost of the half hour at $500,000 per network.
McCain continues to blast Obama for rejecting public funding.
"He wrote down a piece of paper that he would take public financing for his presidential campaign if I would," McCain told ABC News Thursday. "He betrayed the trust of the American people there."