Law professors Michael B. Rappaport and John C. McGinnis explain why the Senate majority's power to modify the filibuster is strongly supported by constitutional principles.

The Senate can choose to retain the filibuster rule, but. . .a majority must be able to change it. The Senate can thereby exercise its full constitutional authority to fashion rules of procedure but past majorities of the Senate cannot put current majorities in a procedural straitjacket. Thus, a change in the filibuster rule by a majority is not a "nuclear" option but instead the constitutional option – the route contemplated by our founding document.

Rappaport and McGinnis also note that the existing filibuster rule itself was a product of a process of an amendment to the rules. Prior to 1975, it took a two-thirds vote of the Senate to end a filibuster. Walter Mondale was one of the Senators who proposed a new rule that would enable 60 Senators to halt debate. The rule was adopted under the threat by Democrats and liberal Republicans to use the constitutional (or "nuclear") option.

Via Real Clear Politics.

The entire history of the constitutional option, including the complicated maneuvering in 1975, is recounted by Martin Gold and Dimple Gupta in the Fall 2004 issue of the Harvard Journal of Law & Public Policy.