As a result, many voters probably gleaned most of their information from media reports rather than campaign ads — news coverage that has been intensely focused on Republican front-runner Donald Trump.

Federal Election Commission reports show that super PACs and other independent groups supporting the Republican and Democratic White House contenders spread $15.4 million worth of television spots and other paid voter communications across the Super Tuesday states through Monday evening. That compares with the $53 million outside groups dumped on Iowa and the nearly $76 million lavished on New Hampshire. (The figures do not include money spent on national TV spots and do not include the amounts spent by campaigns, which do not file comparable disclosures about their state-by-state spending with the FEC.)

One of the major reasons for the dramatic drop-off is former Florida governor Jeb Bush's exit from the race. That brought an end to the gusher of spending by his allied super PAC, Right to Rise USA, which had been the most dominant outside group.

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Without Right to Rise in the mix, the biggest spender is now Conservative Solutions PAC, a super PAC supporting Sen. Marco Rubio of Florida, which invested at least $4.3 million into Tuesday's contests. The group put most of its resources into Texas ($1.5 million), as well as Virginia ($744,318.08), Georgia ($658,762.83) and Tennessee ($407,908.49), where it worked to mobilize upper-middle-class suburban voters in cities such as Atlanta and Nashville.

"Super Tuesday has always been about delegates, and we have targeted our ad spending toward markets where there are a disproportionate amount of delegates available," said Jeff Sadosky, a spokesman for the super PAC.

In the GOP race, Sen. Ted Cruz has benefited from the second-largest amount of super-PAC air cover. Together, five groups spent nearly $2.7 million bolstering him in Super Tuesday states, with a big emphasis on his home state of Texas and on Oklahoma. The majority of the ads have been sponsored by Keep the Promise I, a super PAC financed by hedge fund billionaire Robert Mercer that has been the top-spending pro-Cruz super PAC overall.

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The group has reported spending $11.2 million on the entire 2016 race, slightly more than it raised through the end of January. That's because the super PAC recently received donations from new contributors who like the group's strategy, said Kellyanne Conway, the GOP strategist running Keep the Promise I.

"We are positioning Senator Cruz as the last adult standing," she said. "We have done what super PACs are expected to do through Super Tuesday, which is we spent most of our money helping Senator Cruz."

Several other pro-Cruz super PACs have also invested money into the states holding nominating contests Tuesday, including Stand for Truth ($500,000 opposing Trump in Texas), Keep the Promise III ($249,411 in eight states) and Keep the Promise PAC ($112,258 in seven states).

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One big mystery is the relative silence of Keep the Promise II, which was seeded with a $10 million donation from energy investor Toby Neugebauer last year. The group has reported spending only $704,000, and it has not reported making any expenditures in Super Tuesday states. Neugebauer declined to comment, but a person familiar with the super PAC's spending said it has spent $1 million working to mobilize Cruz voters.

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The third-biggest super PAC playing in Super Tuesday states has been New Day for America, which is backing Ohio Gov. John Kasich. It put in $2.6 million, the vast majority in Massachusetts.

While Trump does not have a sanctioned super PAC supporting him, he is fielding increasing fire from groups opposing his candidacy. Together, the anti-Trump Our Principles PAC, Club for Growth Action and Stand for Truth spent $1.8 million on ads attacking him in Super Tuesday states.