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DENVER — The National Rifle Association will weigh in on three of this year’s key Senate races with a multimillion dollar ad buy beginning Wednesday.

The NRA effort is aimed at boosting Republican Senate challengers in Arkansas, Colorado and North Carolina — the outcomes of which could likely determine which party controls the chamber.

The group’s campaign is expected to top $4 million in these states, spokesman Andrew Arulanandam confirmed. It will spend $1.4 million to boost North Carolina House Speaker Thom Tillis, who is running against incumbent Democrat Sen. Kay Hagan.

“Our Second Amendment rights are under attack by the Obama Administration, and we need leaders like Thom Tillis in the Senate to fight back,” states the ad.

The group will also spend about $1.4 million to run similar ads for Rep. Tom Cotton, R-Arkansas, who is challenging incumbent Democrat Sen. Mark Pryor and about $1 million in Colorado on behalf of Rep. Cory Gardner, running against Sen. Mark Udall.

The group also last month launched several multimillion dollar ad campaigns — one pushing Republican Ed Gillespie, who is up against Democrat Sen. Mark Warner. The other one talks about issues beyond gun rights, trying to reach those who believe the country is on the wrong track as it highlights such issues as the IRS targeting scandal and NSA surveillance.

The group also has run ads attacking former New York Mayor Michael Bloomberg, who has vowed to spend $50 million of his own money this year pushing for more gun control as he and his group Everytown Gun Safety tries to counter the political influence of the NRA.