He cited increased cost for customers (by as much as 25 percent), the loss of U.S. jobs, and less innovation in the telecommunications market.
He also claimed that the merger would create an AT&T and Verizon duopoly that would weaken Sprint so much that it'll eventually be acquired by one of the two.
If the merger were to happen, AT&T will control 43 percent of the market, Verizon will control 39 percent of the market, and Sprint will only control 15 percent of the market, according to Franken's letter.
Since Verizon uses LTE technology and AT&T is committed to LTE technology, it'll likely become the industry standard and put Sprint's WiMAX technology at a "significant disadvantage."
Developers and stakeholders in the telecommunications industry will be hesitant to develop products for Sprint's WiMax. Furthermore, Sprint's 4G phones wouldn't be able to roam on LTE networks. For non-4G roaming, AT&T and Verizon could jack up the roaming rates to push Sprint out of business.
Franken's letter asserts that Sprint's market share will steadily decline until it's acquired by either AT&T or Verizon. When that happens, all the negative effects of the AT&T and T-Mobile merger - the higher prices, jobs losses, and stifled innovation - will be worse.
Below is the full text of Franken's letter.