Only a small fraction of the ads run by outside groups backing one presidential candidate or another have been negative. Voters may not even notice that the sponsor of an ad touting a particular candidate isn’t the campaign itself.

On the flip side, the effect of this gauzy, super PAC-fueled political advertising is questionable.

So far, October’s biggest presidential advertiser, Right to Rise USA, is a super PAC created to boost former Florida Gov. Jeb Bush. It’s aired roughly 45 percent of the TV ads in the presidential race during October — nearly 5,300 ads primarily targeting the early primary states of Iowa, New Hampshire and South Carolina.

The Bush-backing super PAC has now sponsored more ads than any other group or candidate during the Republican presidential primary. By another measure, it’s aired more than one out of every four ads on the GOP side — including three of every five TV ads so far during October.

During the period from Oct. 1 to Oct. 26, Right to Rise aired more than three times as many ads as the second-biggest Republican-side sponsor, the Conservative Solutions Project, a nonprofit supporting Florida Sen. Marco Rubio.

Nevertheless, Bush has consistently languished in polls behind businessman Donald Trump, who has yet to air an ad, and Ben Carson.

Last week, Bush announced a campaign shake-up aimed at cutting costs, reallocating staff to early states, and sharpening focus on New Hampshire, which hosts the nation’s first primary contest in February.

“Plenty of money is being spent, we’re seeing more TV advertising than we have in the past, but the return on investment right now seems to be less,” said Elizabeth Wilner, senior vice president of Kantar Media and the executive who oversees the Campaign Media Analysis Group responsible for the advertising data.

Kantar Media/CMAG monitors television ads that run on local broadcast TV in all 211 media markets, as well as national network and national cable TV. It does not monitor local cable stations or track digital advertising.

Overall, the group that has run the second-largest number of ads on the Republican side remains the Opportunity and Freedom PAC, a now-defunct super PAC that ran more than 3,400 ads in an unsuccessful attempt to boost the flagging candidacy of former Texas Gov. Rick Perry.

Perry withdrew from the race last month. In the wake of his withdrawal, the super PAC suspended operations and refunded contributions to its donors.

According to the Kantar/CMAG data, the only major Republican candidates who have aired any TV ads directly so far this cycle are Carson, Bush, New Jersey Gov. Chris Christie and U.S. Sen. Ted Cruz of Texas.

Michael Beckel contributed to this report

This story is from the Center for Public Integrity, a nonprofit, nonpartisan investigative media organization in Washington, D.C. Read more of its investigations on the influence of money in politics or follow it on Twitter.