It's the "tea party" vs. the Republican Party in Tuesday's Senate primary in Delaware, where a popular moderate is suddenly under siege from a little-known conservative who in any other year might have been relegated to the footnotes of 2010's election records.

That's not an entirely unfamiliar narrative in a year in which tea party organizations have ousted two incumbent senators. But Christine O'Donnell's battle with Rep. Mike Castle perhaps embodies the movement's greatest test, because unlike in other races in which the GOP has offered the tea party an awkward embrace, the Republican Party is fighting back.

Read the entire Washington Post article here.