The Republicans are, if anything, as much in revolt against the establishment within their own party as they are against the Democrats. And this is what the Tea Parties (which should always be referred to in the plural, because they are not a monolithic movement) are all about: they are not just a reaction against a left-liberal president but a repudiation of the official opposition as well.
The ascendancy of the Tea Parties has meant that fiscal conservatism can replace social conservatism as the raison d'être of the Republican cause. So rather than being a threat to Republicanism, the election of Tea Party candidates might be its salvation. It represents a rank-and-file rejection of what many Americans see as a conspiracy of the governing elite against ordinary working people.
All of which makes clearer the appeal of even the naivety and inexperience of some of the Tea Party contenders who have challenged incumbent Republican candidates. If what you are rebelling against is a generation of smug, out-of-touch professional politicians, then a little dose of amateurishness or innocence might strike you as positively refreshing.