Seven U.S. states have joined the U.S. Department of Justice (DOJ) lawsuit to block AT&T's $39 billion proposed merger with T-Mobile.

The states are California, Illinois, New York, Pennsylvania, Ohio, Massachusetts, and Washington.

At the heart of the matter is anti-competition; in an already concentrated industry, the AT&T T-Mobile deal would shrink the field from four national players to three.

Moreover, the combined AT&T T-Mobile entity would hold a whopping 43 percent of the market share, giving it a commanding lead over its three smaller competitors. 

 T-Mobile has been an important source of competition among the national carriers...Unless this merger is blocked, competition and innovation will be reduced, and consumers will suffer, said Sharis A. Pozen, Acting Assistant Attorney General in charge of the Department of Justice's Antitrust Division.

Many of the seven states that have joined the DOJ lawsuit also cited anti-competition as their concern. 

Below is what their attorney generals have said:

New York: This proposed merger would stifle competition in markets that are crucial to New York's consumers and businesses, while reducing access to low-cost options and the newest broadband-based technologies.

Ohio: The proposed transaction threatens to substantially lessen competition for mobile wireless providers across the United States, resulting in poorer quality services, fewer consumer choices and ultimately, higher prices.

California: Our review of the proposed merger between AT&T and T-Mobile has led me to conclude that it would hinder competition and reduce consumer choice.

Illinois: Blocking this acquisition protects consumers and businesses against fewer choices, higher prices, less innovation, and lower quality service.

Washington: This merger will result in less competition, fewer choices and higher prices for Washington state consumers.  If the deal goes through, two companies will control roughly three quarters of mobile subscribers in the U.S. Antitrust laws exist to prevent such strangleholds over products and services.

Massachusetts: The proposed merger would create highly concentrated markets in Massachusetts and could lead to higher prices and poorer service.  Competition will best serve consumers and businesses in Massachusetts who rely on mobile wireless services in their everyday lives.