GOP's war for eyeballs

Republicans are starting to trounce Democrats in political spending across the Senate map, and that advantage is now showing up in one of the metrics campaigns care about most: impressions.

Why it matters: Money matters in a campaign's closing weeks, but what campaigns are ultimately buying is voters' attention — and the price varies, depending on who is buying and when they place their ads.

Beginning in September, campaigns and super PACs start competing for the same finite supply of advertising inventory.

But the campaigns are guaranteed lower rates.

Republicans are generating substantially more impressions — an estimate of the number of times an ad is seen — but their advantage isn't nearly as large as the spending disparity would suggest.

The intrigue: North Carolina is the only key battleground where Republicans have been spending more for fewer impressions in the first weeks of September.

Democrats generated 245 million impressions supporting former Gov.

Roy Cooper, compared with 226 million for Republican Michael Whatley— even while being outspent by Republicans $10.6 million to $9 million.

Whatley has been consistently trailing Cooper in the polls.

It is forcing Republicans to weigh whether to begin pulling resources there.

One key GOP super PAC took down $6.5 million worth of ads for October, as Semafor first reported .

By the numbers: From Sept.

1 through Sept.

17, Republican advertisers generated 502 million impressions in Texas, compared with 331 million for Democrats, according to data compiled by Kinetiq Political Insights.