Peggy Noonan explains it perfectly in a WSJ column you can read here.
I don't consider myself a Tea Party guy. I'm a conservative, and the Republican Party has been the conservative "vessel" for most of my life. When the Obama administration's excesses began to give rise to the Tea Party folks, I found it encouraging that people were finding their voices, but I thought it would be a short-lived movement. Now I'm not so sure.
People are genuinely angry - as well as worried - and while they are REALLY unhappy with Obama, they are also suspicious of Republicans who don't want to be real conservatives. Delaware's senate primary was a great example. Four years ago, Mike Castle would have cruised to the nomination, but for many of us on the right side of political spectrum, it's no longer enough to call yourself a Republican and then vote with the liberals a good chunk of the time. If you want the Tea Party backing, you better be willing to say "no" loudly and often.
Conservatives, as Peggy explains, are looking for Republicans who don't start negotiating with the Democrats in the middle, and compromise halfway to the total liberal position. We want people who will start the negotiations on our end, and go no further than halfway.