That's why they call it "illegal" coordination:
In a joint letter to FEC Chair Caroline Hunter and Vice Chair Ellen Weintraub, the lawmakers — led by Senator Sheldon Whitehouse of Rhode Island — asked the FEC to “open an investigation into a potential campaign finance violation” alleged by the Campaign Legal Center, a watchdog group, in two complaints that are currently before the commission. The complaints claim that the NRA uses a company called Starboard Strategic Inc. to circumvent laws prohibiting election-related coordination between campaigns and outside groups who support them.
Prior to the creation of Starboard in 2013, the NRA used OnMessage as a vendor to place political ads. Beginning in the 2014 election cycle, the group shifted to Starboard, spending millions of dollars for ads supporting the campaigns of three Republican Senate candidates: Thom Tillis in North Carolina, Cory Gardner in Colorado, and Tom Cotton in Arkansas. All three campaigns paid OnMessage as a consultant, and all three won.
Keep in mind, this was going on at the same time Tillis (and drunken Dallas) were using Russian-backed Cambridge Analytica to conduct a personally-targeted and invasive propaganda scam to trick voters into voting for one of North Carolina's emptiest of suits. Since Tillis is not running in this cycle, it might seem like an issue that could wait. But they're pulling the same shenanigans in a couple of 2018 Senate contests: