From an answer to another RedState poster (with an edit at the end):
To answer you straight, a reason conservatives have a hair trigger when it comes to their elected favorites is that it matters so much. If you are a member of a sizable majority, the occasional defection for whatever reason can make no difference at all; it can be understood as a tactical move prior to an election, important to the defector but not to the final outcome.
The same is true if you are part of a hopeless minority. Olympia Snowe and Susan Collins were acceptable for that reason, for a while. They usually voted the right way, seldom made any difference. The most important thing they did was fill Senate seats as Republicans, giving the 'Pubs two vote closer to a majority for CONTROL OF THE SENATE or to block Democrats as a significant minority.
When legislative things get close, then individual votes start to matter more. In those situations, Democrats seem to understand and they toe the line. Republicans don't, perhaps because they haven't had enough experience as a majority. Bart Stupak was the poster boy for being "persuadable." He and his constituents were anti-OCare-abortions, so he was promised there would be none, and his vote pushed OCare over the top (IIRC). It turned out that his constituents were simply anti-OCare. He declined to run for re-election in 2010, and in gratitude his constituents elected a Republican to replace him. So toeing the line can have its price, as can broken promises. And now we have an OCare that includes abortion funding in some cases.
But I digress. Mis-steps by Rubio and Rand Paul and anybody else are magnified in importance because of the size of their footprints. To use Rubio as an example, he was elected by a popular groundswell in reaction to obvious dissembling by Charlie Crist. He was, and IS, a solid conservative--about everything but the immigration issue. Therefore, I could forgive his position on it, even his participation in the Gang of 8, UP TO A POINT.
As long as he was steadfast in his demand for sensible sequencing, for border security before amnesty (let's just use the word instead of being nit-picky about whether it's the "right" word or not), as long as he was truthful, he got a pass from me because of his unique situation. Once he started backing off of security-first, claiming that a plan is as good as a deed, accommodating Chuck Schumer in back-room deals, he lost my support. As I have stated here more than once, he can regain it only by renouncing the Go8 deal and removing his name from it.
Face it. If Rubio's name were not on this abomination of an immigration disaster, it wouldn't have much of a chance of getting out of the Senate, let alone passing in the House. His support for it is crucial. And his opposition would be crucial as well.
So let that be an answer to your unasked question--Why are conservatives so quick to denounce their former favorites? Because they're so important, but only as long as they remain conservative on critical issues.