Josh Miller / CNET

The Department of Justice may have signed off on T-Mobile's $26 billion merger with Sprint, but state attorneys general are still trying to block the deal. Oregon on Monday joined 15 states and the District of Columbia in a lawsuit that seeks to halt the bid, calling it an "anticompetitive megamerger."

The suit was filed in June by several state attorneys general, led by New York and California. They argue that the merger will harm competition and will cost their residents $4.5 billion annually.

"Oregon's addition to our lawsuit keeps our momentum going, and ensures that there isn't a single region of this country that doesn't oppose this anticompetitive megamerger," New York Attorney General Letitia James said in a statement Monday.

Sprint declined to comment. T-Mobile didn't respond to request for comment.

The Justice Department approved T-Mobile's bid to buy Sprint, which was first announced more than a year ago, last month. The deal was also approved by the Federal Communications Commission in May. While these were major hurdles to clear, T-Mobile said previously that it wouldn't proceed with the closing of the deal until it settled the concerns of the state AGs.