Is what MoveOn claiming correct? That Gallup is basing their polls on Rep. turnout of 40%? If that is the case, I can see where MoveOn would have a point.
It's based on voter/polling respondent self-identification. The real news is that more people polled are starting to identify themselves as GOP. This is contrary to past elections, but movements like this have taken place in this country's history.
My guess? The shrillness of the Dems and their surrogates are turning a lot of average Joe/Jane Democrats against the party. It's what happens when the kooks run any party. The silent majority starts to silently migrate somewhere else.
It is common that after a convention or a big news event to see such drastic changes from established levels. That is one reason to ignore the first round of polls taken after a convention.
It is not common for it to have endured, the way this has. Will it endure through the election? I do not know. Will it correspond with huge Republican pluralities turning out? I do not know.
But what I do know is that Gallup, ABC News/Washington Post, CBS News, Pew and others are measuring something. They are not making it up.