MetroPCS today said it is proposing a merger with Leap Wireless, forming what it claims will be the fifth nationwide cellular operator. In a letter sent to the Leap board, MetroPCS Chairman and CEO Roger Linquist offered to exchange 2.75 shares of Metro stock for every share of Leap stock in a deal worth $5.5 billion.

The merger, in many ways, is the perfect fit for both companies. Metro focuses on large metropolitan markets like San Francisco and New York, while Leap targets the middle of the U.S., launching originally as Cricket in mid-sized markets in Tennessee and expanding into the South, West and Midwest from there. In addition, both companies follow a similar business model, offering all-you-can-eat minute plans for local calling and charging for long-distance separately. In fact, Metro’s use of such calling plans was the subject of a lawsuit Leap filed last month, alleging Metro stole its patented business model.

The combined carriers would have 6.2 million customers (3.5 million from Metro and 2.7 million from Leap), putting it far behind the nationwide providers, the smallest of which (T-Mobile) has 27 million. The merged company wouldn’t even be the largest of the Tier II providers: Alltel has 12 million subscribers, and U.S. Cellular would be only a hair smaller with 6 million subs. There is something to the Metro claims, though, that combining with Leap would produce the fifth nationwide carrier. Both of the companies own significant spectrum, much of it acquired in the recent Advanced Wireless Services (AWS) auction, and both of them appear to have aggressive expansion plans.

“First and most important, the combination of our two companies would create a new national wireless carrier with licenses covering nearly all of the top 200 markets in the United States,” Linquist wrote in a letter addressed to Leap Chairman Mark Rachesky and President and CEO Douglas Hutcheson. “Such a combination would significantly expand the network service area available to the subscribers of both companies and would better position the combined company to more aggressively compete with the other national wireless carriers.”

Leap’s most ambitious plans lie in the Midwest where it won an AWS license covering the entire Great Lakes and Midwest regions, putting major metro areas like Chicago directly in its footprint. MetroPCS used the AWS auction to expand from the west coast to the east, winning key licenses in the Northeast, in particular a much-coveted license covering New York City.

Officials representing Leap said this morning the company was not yet commenting on Metro’s offer.