Changing rules with a simple majority vote is considered so controversial it is sometimes called the nuclear option. Democrats backing the maneuver have described it as the “Constitutional option.”And TPM:
Changing the rules of the Senate ordinarily requires 67 votes. But the majority also has the option of approving rules changes with 51 votes at the beginning of a new Congress — what reformers call the “constitutional option” and opponents dub the “nuclear option.”Neither of these is correct. The Constitutional/Nuclear Option is a risky in-session parliamentary maneuver primarily anticipated in the event of a filibustered nominee (somebody the President and majority party want to make a hill to die on). The maneuver involves appealing to the Parliamentarian for a ruling on the constitutionality of the filibuster rule (the argument being that the ability of the minority to filibuster a nominee is incompatible with the "advise and consent" clause). Upon a negative ruling, the rule is voided and a new rule is put in place without the offending provision, which most expect to only need a simple majority to pass.
What is being proposed now, by contrast is a perfectly usual first-day-of-session change in a rule, which never requires a two-thirds majority. The operative principle is that an earlier Congress cannot bind later Congresses to any course of action: the original adoption of rules was by simple majority, so changes of rules "in the normal order of business" (interpreted to mean "at the beginning of a session") cannot require a greater majority than the original adoption.
This is complicated by the fact that, while the House gets refreshed every two years, the Senate thinks of itself as a continuing body, and senators who oppose any particular rule change are fond of arguing that there is no true opening day on which to change rules. Which, if you look at the history of this argument, is always deployed in the most rankly opportunistic fashion.