I think Cantor is dead wrong, but what he said was clearly within the bounds of accepted discourse. (See American politicians openly siding with Britain before US involvement in both World Wars).
A more apt analogy perhaps, would be a politician saying that Roosevelt should have done lend lease with the Nazi's rather than the Brits, perhaps. Yes, that is not treasonous either, and no, I am not comparing Israel to the Nazis. I don't think anyone per se complains about loyalty, where the assertion is that we should take a side in some foreign conflict, rather than not.