This is a provocateur seeking to have their enemy throw gas upon a fire. A "shrill ... silence" means silence about the issue, which logically means that the GOP, as a group, declines to do more than what Sen. McConnell has already done.
As Thomas Moore said, "qui tacet consentire videtur"; he who is silent seems to consent and in this case the consent is obviously for the majority opinion - you HEINOUS ASSES. Never take advice from your enemies, your friend's advice is bad enough!
I believe a shrill silence in this sense means their reluctance to condemn the protesters,to speak up in favor of the mosque is actually screaming their opposition to it. In effect applying an action to inaction.