The University of Wisconsin Advertising study found that nearly 100 percent of McCain's ads run between Sept. 28 and last Saturday were "negative," compared to 34 percent of Obama's television spots, The Hill reported Wednesday. For the same period, the Illinois senator spent just under $17.5 million.
So far this election cycle, 73 percent of McCain's ads and 61 percent of Obama's ads have been considered negative by the Wisconsin study.
Obama's surge in the polls coincides with his campaign's outspending McCain in battleground states, the Washington political publication said.
"Ten of the 15 states where both candidates are advertising were won by Bush in the 2004 election," said Ken Goldstein, professor at the University of Wisconsin-Madison and director of the Wisconsin Advertising Project. "The campaign is being played on the Republican side of the field this year."
The two candidates spent most of their advertising dollars in Indiana, Iowa, Michigan, Minnesota, Ohio, Pennsylvania and Wisconsin.