Nasdaq just went nuclear.

In a letter to the US Securities and Exchange Commission, Nasdaq's law firm, Gibson Dunn, said that the SEC could be sued if it approves IEX's application to become an exchange.

IEX, the upstart trading venue made famous by Michael Lewis' book "Flash Boys," filed with the SEC in September to become a stock exchange, kicking off a heated debate of the merits of its application.

The debate has at times gotten ugly, with IEX and the New York Stock Exchange in particular trading barbs.

For its part, the SEC has delayed its decision on the application until mid-June.