I think there are good arguments to be made for why a majority vote of the Senate should not be the process by which we find a rule (or procedure for setting rules) constitutional or unconstitutional. Personally, I think the weight of the reasoning cuts the other way, since there has to be some method to judge the constitutionality of Senate procedures. The Courts have said they will not get involved in any but grotesque procedural situations (See U.S. v. Nixon), but they have also not precluded others from making constitutional decisions. In fact, the logic of Marbury, by which the Court finds its need to assess constitutionality as incidental to live cases, would support (I believe) a role for the Senate itself in judging the constitutionality of its own procedures. Roberts Rules of Order are not the Supreme Law of the Land; only the Constitution demands such perfect adherence.
But beyond that difficult argument, the most fascinating liberal claim has been that somehow the Republicans were out of line for trying to sabotage an institution which by different accounts has existed since Wilson administration, the Civil War, the Founding of the Republic, and the Seven Days of Creation (to be referenced for hyperbole's sake on the condition that they are never mentioned in a classroom). How Delightful! Precilla Owen is opposed because she is so extreme that she thinks the constitutional tradition of not recognizing unlimited personal rights under the Fourteenth Amendment has some salience. But the same attacked-reasoning used by conservatives from Justice Scalia to Embattled Federalist Law Students to interpret the constitution is inscribed on the Filibuster Golem to preserve its life. What a wonderful trick!
The battle between Justice Scalia and Justice Kennedy in Lawrence v. Texas is a rematch of the debate in Michael H. v. Gerald D., which is itself just another chapter in the long line of cases dating back to the Slaughterhouse Cases. If written constitutions are to retain the power we want them to retain, they must be static instruments to bind us from moving too far with majoritarian processes which can be overly susceptible to dynamic and transient impulses. To know the limit of this bind, then, we turn to tradition, since the bind itself is static and not dynamic (that being its only advantage over the otherwise highly legitimate output of democratic majoritarian politics). To understand the limits on the powers of our elected officials, then, we turn in part to history and tradition to see what has been history's understanding of the limits to our legislative powers.
The liberals misunderstand this principle on two fronts now. First, they seek to apply it to Senate rules, whereas it only makes sense in the context of written constitutions. We may have many tax traditions also, but that doesn't mean that attempts to change our tax brackets should be treated with any less deference than the initial creation of tax rates. More critically, though, their conception of rights, and liberties, and freedoms is too dynamic. They see the Constitution as the source of power for mini-legislatures, staffed only with the elite. Its traditions are irrelevant, since it is merely a medium for power for their designated leadership. (This is the Mike "Czar of the Telestrator" Fratello School of Constitutional Jurisprudence. If you have a bad or outmatched team, you try and shorten the game, expecting a greater likelihood of victory in a 30 possession basketball game than in one with 75 possessions. Thus, the liberals---having been convinced that they cannot win in most states or most Congressional districts or most presidential elections---seek to shorten the game by deciding critical matters of society, culture, and law in a nine-person forum). Therefore, they not only don't value traditional interpretations, they need to affirmatively reject them so as to capture power.
But, at least their endorsement of tradition in the battle to preserve a Senate Procedural maneuver must clear the way for future Supreme Court nominees who similarly value looking to our legal traditions to guide our legal present. It must, right?