Republicans have an advertising advantage in four key Senate races after a recent influx in spending from outside groups, an analysis of political spending has found.

More ads have been aired in Alaska, Colorado, Iowa and Kentucky for Republican Senate candidates than Democratic candidates over the past two weeks, according to a new report from the Wesleyan Media Project that looked at advertising in the states between Sept. 12 and 25.

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That's movement from the group's report two weeks ago that found Democrats had so far outspent Republicans in nine of the 10 most competitive Senate battles, trailing only in Alaska.

A nearly $1 million spending advantage by pro-GOP groups over the two-week period has helped eliminate the gap on the number of ads run in North Carolina for Sen. Kay HaganKay Ruthven Hagan10 under-the-radar races to watch in November The Hill's Campaign Report: Democratic Unity Taskforce unveils party platform recommendations Democrats awash with cash in battle for Senate MORE (D) and House Speaker Thom Tillis (R). Two weeks ago Hagan benefitted from nearly 950 more ads running in the state than Tillis.

In Iowa, which saw about equal advertising between the sides as of two weeks ago, ads favoring Republican Joni Ernst now outnumber those for Rep. Bruce BraleyBruce Lowell BraleyThe Memo: Trump attacks on Harris risk backfiring 2020 caucuses pose biggest challenge yet for Iowa's top pollster OPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP MORE (D) by more than 1,500 spots. Of the top 10 races, Iowa has the largest percentage of ads sponsored by outside groups, at 62 percent.

Independent groups sponsored a majority of ads for Senate Minority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellTrump selects South Carolina lawyer for impeachment trial McConnell proposes postponing impeachment trial until February For Biden, a Senate trial could aid bipartisanship around COVID relief MORE (R-Ky.), helping to reverse the single-digit advertising edge for Democratic challenger Alison Lundergan Grimes. 

More than 50 percent of ads run in Colorado have supported Rep. Cory GardnerCory GardnerOvernight Defense: Joint Chiefs denounce Capitol attack | Contractors halt donations after siege | 'QAnon Shaman' at Capitol is Navy vet Lobbying world Senate swears-in six new lawmakers as 117th Congress convenes MORE (R-Colo.), a major swing from incumbent Sen. Mark UdallMark Emery UdallHickenlooper ousts Gardner in Colorado, handing Democrats vital pickup Live updates: Democrats fight to take control of the Senate The 10 Senate seats most likely to flip MORE (D-Colo.)'s 16-point advertising advantage two weeks ago. There, the majority of pro-GOP ads came from outside groups.

Republicans have also cut into the advertising edge for Democrat Michelle Nunn in Georgia, from 32 percent of total advertising to 44 percent going toward Republican David Perdue.

It is not all bad news for Democrats, however. Spots supporting Sen. Mary LandrieuMary Loretta LandrieuCassidy wins reelection in Louisiana Bottom line A decade of making a difference: Senate Caucus on Foster Youth MORE (D-La.) over Rep. Bill Cassidy (R-La.) have grown from 57 percent to 65 percent of total advertising.

Alaska has remained relatively unchanged, with 52 percent of ads favoring Republican Dan Sullivan over Sen. Mark BegichMark Peter BegichAlaska Senate race sees cash surge in final stretch Alaska group backing independent candidate appears linked to Democrats Sullivan wins Alaska Senate GOP primary MORE (D-Alaska).

More than 40 percent of pro-Republican ads in Alaska, Colorado, Iowa and Kentucky were sponsored by outside groups, based on an analysis of ads run between Sept. 12 and 25.

This post was updated at 1:21 p.m.