Ronald Reagan was one such democrat ‘defector’ although he ably characterized the ‘defection’ as “I did not leave the democrat party, that party left me”.
How would you categorize Rudy Guiliani ? He was a liberal democrat that got sick of the machinations of the party, he was not then, nor is now, a conservative. Never the less, Reagan saw fit to make him a U.S. Attorney and supported him there.
My personal opinion, is that while Rudy isn't a "Rat Infiltrator", he is absolutely a liberal who would love to turn the GOP to the left, like Chris Christie...to many people mistake partisanship with ideology.
Rudy is no conservative, but while he may be very partisan (at least now) with the GOP, he sure as hell is liberal (In NYC, he ran on not only the republican partys line, but also the now defunct Liberal party ballot line).
Guiliani was a Reagan democrat, he was never a genuine Republican. He ran against Dinkins twice; the first he lost and the second time he won only because Dinkins stunk so bad that even the most ardent base democrats had a tough time supporting Dinkins.
Was Guiliani a Rat Infiltrator to the GOP? Yes and no. He went from democrat (Carter) to independent (Ford) to republican (Reagan). He went to GOP only to further his career as prosecutor and because Reagan was so appealing.
Yes he was an infiltrator because he used the GOP brand in much the same way that Bloomberg used it, for his own ends. He was never a Republican in ideology, that he believed in limited government, in strict interpretation of the Constitution.
No he was not an infiltrator because of his appeal within the GOP as adhering to a philosophy of the Rule of Law.
So he is difficult to classify but I would say that under a Conservative Revolution of the type that rolls back the New Deal, rids the USA of the LBJ Great Society, discredits the Compassionate Conservatism of George W. Bush and seeks to appoint unequivocally conservative justices, I would say that Guiliani would not participate as a GOP ideologue, so that the GOP would leave him and he would be nowhere except as a New Yorker in NYC affairs.