As disclosed spending by third-party groups crests $650 million, it’s voters in a handful of states who are bearing the brunt of the advertising barrage. In Alaska’s Senate battle, where incumbent Democrat Mark Begich is running neck and neck with Republican Attorney General Dan Sullivan, we estimate that outside groups have spent $120 — or more — for each likely voter in the sparsely populated state.

We used the Federal Election Commission’s turnout data from the 2012 presidential election as an estimate of the number of voters in each Senate contest. Voter turnout in presidential elections is always higher than in midterms, so it’s likely that our estimate of spending per vote is conservative.

The Senate race in Alaska, in which the Rothenberg Political Report gives Sullivan the slight edge, has attracted money from the biggest campaign players in the country, including the Senate arms of the Democratic and Republican parties, the conservative powerhouse American Crossroads and Senate Majority PAC, which has been funneling money to the pro-Begich Put Alaska First super PAC.

In all, that state has seen some $36 million in independent expenditures reported to the Federal Election Commission. That’s a lot of outside interest by any measure, but it’s even more impressive considering the state’s population: 730,000, approximately the same number of people who live in Charlotte, N.C. The actual amount of outside groups have spent is doubtlessly higher than the reported figure. That’s because “issue” ads that air more than two months before an election and stop short of directly urging a vote for or against a candidate don’t have to be reported at all.

The wave of federal spending was drowning out campaigns further down the ballot. National Journal reported Oct. 23 that GOP gubernatorial candidate Sean Parnell’s campaign finds itself on the sidelines of the TV air wars, as the remaining spots have already been snatched up by the national parties or super PACs or are prohibitively expensive thanks to the advertising arms race.

Political ad contracts collected by Political Ad Sleuth via the FCC show 30-second ad spots at local NBC affiliate KTUU in Anchorage — where about 40 percent of the state’s population resides — in mid-October going for as much as $3,900 a pop for 30 seconds of prime air time.

And while the Last Frontier State is the most striking example of big outside money drowning voters in ads, it’s far from the only one.

Voters in New Hampshire, Arkansas and Iowa — all of which are in the midst of competitive Senate races — have likely seen more than $30 spent by outside groups for every individual voter.

That’s not good news for people bemoaning the growing negativity and polarization of politics: Third-party ads are overwhelmingly negative, and there’s undoubtedly more to come. With one week left until Election Day, the sea of special interest money will undoubtedly keep rising.