What are arguments that Orthodox Jews have published (not on Mi Yodeya) that Conservative Judaism is an invalid form of Judaism?
Have any Orthodox authors written articles or books that made such arguments in detail?
I'm going to paint this in overly broad strokes, but here goes:
Theologically, the official stance of the Conservative movement is that the Torah was "inspired by God, but written by man." Orthodox theology believes this to conflict with one of Maimonides' principles of faith, that the Torah was dictated by God word-by-word to Moses.
An early practical demarcation between Orthodox and Conservative synagogues was the presence/absence of the mechitza, with many Orthodox rabbis opining that it was prohibited to pray at a synagogue where the men and women sit together. (Much ink has been spilled on this, and at one point a major Orthodox seminary issued all its graduates a copy of The Sanctity of the Synagogue, a book all about this subject.)
On various other matters, Orthodox scholars have felt that Conservative rabbis' readings of halacha have been "overly creative" in the service of whatever conclusion is desired, put mildly.
... the responsa of other denominations. I have seen quite a few (they used to be far more popular decades ago, when they made more of a pretense of being halachic) that follow a predictable sequence:
1) Decide what conclusion you want to arrive at. This will often be based on predicting what the Jewish ethical response must be in a world that has changed so significantly from the early legal texts of Judaism, that the modern author is given much leeway.
2) Find a few gemaros that seem to deal with the issue. If they don’t agree with your conclusion, either ignore them altogether, or find some understanding of each counterexample which will make it irrelevant to our times. This can be done by finding a single Rishon whose explanation of the gemara makes it possible to argue that the rabbis of the Talmud simply would not have said the same thing today. It doesn’t matter if that Rishon’s thinking is outweighed by a huge number of contradictory opinions.
3) Alternatively, show why such thinking is simply at odds with contemporary insight and reasonableness, and therefore must be discarded as foreign to the “spirit” of Jewish law and its inherent resiliency and flexibility.
4) Find a medrash as a springboard to show how quintessentially Jewish, how much in the spirit of Jewish law your own conclusion is.
5) Accept your original argument.
Or as our friend Fred put it in a comment below (thank you mori urebbi):
The common thread in those examples is that the Conservative movement (broadly speaking) purports to be a halachic movement but they employ untenable halachic arguments to suit a desired outcome that is itself based in extra-Judaic (or even contra-Judaic) ethics or values.
Here are some nice illustrative examples:
Driving a gasoline-powered car on the Sabbath: (Rabbi Adlerstein gives this example.)
Orthodox: The Torah says don't burn things on Saturday, therefore you can't drive a car as it burns gasoline.
Reform: The Torah is wrong. The point is to rest, and burning things is only a lot of work when you have to rub stones together. Driving is a much better way of resting than walking a mile!
Conservative: Well ... the Torah says don't burn things, but ... well ... the Talmud says it's not as bad if you do it for different purposes than its Biblical use. The Biblical use of burning was to boil water [to cook dyes for tapestries], but when you burn gasoline the purpose is for transportation. (Orthodox response: the Talmud's example of alternate-purpose is burning a document to destroy it. The Biblical use case is burning as a fuel source, which is exactly what a gasoline engine does.)
Intimate touching between two men.
Orthodox: The Torah says men shouldn't sleep together. It also says "don't come close to shameful acts"; just as it's wrong for a married woman to do "everything but" with someone other than her husband, it's wrong for two men to do "everything but."
Reform: The Torah is wrong. Religion should have no say on what consenting adults do with their private parts.
Conservative: Well ... we can't contradict the verse in the Torah ... so we can't actually allow certain actions ... but ... well we'll just have commitment ceremonies, we don't know what they'll really do in private ... and because of human dignity, we'll allow "everything but." (Orthodox response: "human dignity" is used Talmudically to allow minor infractions in situations where they've accidentally collided with human dignity, e.g. there's no pre-cut toilet paper in the bathroom and now you're stuck. Not to do away altogether with a prohibition.)
A female cantor reciting the Amida out-loud, and a man in the congregation will listen to fulfill his obligation
Orthodox: the Talmud says you can only fulfill someone else's obligation if your level of obligation is greater than or equal to theirs. The Talmud also says that men have more obligations vis-a-vis prayer than women; therefore, a woman praying out-loud can't fulfill a man's obligation.
Reform: The Talmud is wrong. There should be no gender differences in Judaism. What's more, the whole concept of a monarchical deity dictating is outdated. Prayer should respect personal autonomy and individual spirituality.
Conservative: well, we can't argue with the Talmud ... but ... well, we can demand our female cantors take a vow to pray thrice daily, thus reaching the same level of obligation as men. (Orthodox response: that obligation would be due to the force of the vow, not the original obligation to pray per se.) Or ... we will hereby obligate all women to pray thrice daily, and if most of them don't or can't do so, well, that's not our concern.