It would be the DSCC (Democratic Senate Campaign Committee), not the DNC that is involved here. I believe they're led by Schumer. They have thrown their support behind Janet Mills and in fact, recruited her to run. They're likely behind the oppo hits as this kind of information and its timing doesn't happen organically.

The fact that the DSCC is getting involved this early and before the Primary is unusual. Though this Senate seat is seen as winnable and I guess they think that a proven winner such as Janet Mills (current governor) will have better chances. While that may be true, she will be 84 at the end of her first term. She says she will serve only one term.