Voters head to the polls for primaries in six states today, with those in Arizona, North Dakota and Virginia inundated with the most money from super PACs trying to influence their votes. Here's a rundown of what outside groups are spending in the Grand Canyon, Peace Garden, and Old Dominion states, according to Sunlight's Follow the Unlimited Money tracker: 

Special Election: Arizona's 8th Congressional District

Among today's races, the special election to fill the remaining term of Rep. Gabrielle Giffords, D-Ariz., who resigned in January to focus on her rehabiliation, has attracted the most outside money, more than $2.3 million. The majority of that—$1.4 million—has been spent to benefit Republican Jesse Kelly, a former Marine. The independent cash just about equalizes the air war, since his opponent, former Giffords aide Ron Barber, had raised about $400,000 more than Kelly as of late May. The blitz opposing Barber comes from the National Republican Campaign Committee, American Crossroads and the Arizona Republican Party, while groups like the American Action Network have spent resources on pro-Kelly voter calls. The NRCC TV ad calls him "rubber stamp Barber" for supporting "more failed Obama policies."

The Democratic Congressional Campaign Committee and the House Majority PAC  have been countering the Kelly barrage against Barber, spending about $9 million for, among other things, a new ad painting Kelly as an extremist. 

Unlike many of the primary contests this year, hard money has been king in the special election, with over 50 percent of the outside money coming from party committees—mainly the main parties' national campaign committees. Those organizations face strict limits in the amount they can raise from each donor, as opposed to super PACs. 

North Dakota Senate

In North Dakota, Republican Rick Berg, the state's sole representative in the House and former Democratic gubanatorial candidate and attorney general Heidi Heitkamp, are their respective parties' favorites; Heitkamp, in fact, is running unopposed. A super PAC focused on maintaining the Democratic advantage in the Senate called Majority PAC just bought a TV ad—for nearly $70,000—painting Berg as a party-line voter in independent thinking North Dakota. A new poll shows Heitkamp with a slight edge over Berg in the Republican-leaning state. Berg, backed by the GOP establishment, faces former Navy captain Duane Sand in the primary. 

North Dakota's At-Large Congressional Seat

With Berg vying for a Senate seat, there is now a contested race to be the GOP nominee to represent Nebraska in the House. The battle is between two state public service commissioners, the establishment-backed Brian Kalk and Kevin Cramer, supported by anti-tax group Club for Growth. The Club's super PAC dropped about $55,000 on pro-Cramer mailers, and two other super PACs— the Tea-Party group FreedomWorks and the Life and Marriage PAC—have spent thousands of dollars backing him. Both the Kalk and Cramer campaigns had raised about the same amount of money—$300,000—as of late May.

Virginia Senate

It's also worth mentioning that the Senate race in the battleground state of Virginia, one of the this year's most closely watched races, has been attracting outside groups for quite some time. Former governor and Democratic National Committee Chairman Tim Kaine will most likely be facing off against former governor George Allen, who lost his Senate seat to Jim Webb in the 2006 election, as both are expected to easily win their parties' nominations today. But the general election fight has been building steam for a while. The nonprofit Crossroads GPS, which does not have to disclose its donors or its ad buys because they criticize candidates without calling for their defeat, has been running ads critical of Kaine since last year, including a November ad that called Kaine a "reckless spender" and "Obama cheerleader" and a similar April spot.  

Meanwhile, almost all of the independent expenditure ads (those calling on citizens to vote for or against a candidate) have come from left-leaning super PACs and nonprofit groups supporting Kaine—to the tune of about $300,000. The most significant was a May Majority PAC ad criticizing Allen for increasing spending as governor and praising Kaine for his fiscal responsibility.