WILMINGTON, DEL. - 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.

The reason, state GOP officials argue, is that O'Donnell is simply unqualified to hold office. A two-time losing candidate against then-Sen. Joseph R. Biden Jr. - whose old seat she and Castle are seeking - O'Donnell has a history of financial difficulties documented in the Wilmington News Journal that includes an IRS lien, a near-foreclosure on her mortgage and a dispute with her alma mater for not paying college expenses. She has a tiny campaign operation and, for most of the year, she received virtually no grass-roots financial support within Delaware.

What O'Donnell has is the backing of Sarah Palin, Sen. Jim DeMint (R-S.C.) and the Tea Party Express. Their support has put her on the national political map not to mention within striking distance of upsetting Castle on Tuesday.

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Her Republican critics say she would be almost certain to lose, in moderate Delaware, against Democrat Chris Coons in November. That raises the stakes for those in the tea party who are bent on reforming the GOP, even if it means losing seats. It also raises the question of whether the uncomfortable alliance between the tea party and the GOP is coming to an end.

"It is just unbelievable that you would have folks from out of state who know nothing about Delaware, nothing about Christine O'Donnell and nothing about the dynamics of this state to come in and associate themselves with this person," said Tom Ross, chairman of the Republican Party of Delaware and one of O'Donnell's most outspoken critics.

"Quite frankly, I don't know what the goal is here," he continued. "If they really want to see conservatives as chairmen of committees in the United States Senate, they'd step back and allow Mike Castle to become the next United States senator. He'll win the general election. She won't."

Ross thinks Castle will win, and Castle himself said in an interview on Sunday that his internal polling shows that he is "fine." But they are taking nothing for granted in a year when Sen. Robert F. Bennett of Utah and, more recently, Sen. Lisa Murkowski of Alaska, were blindsided by angry tea party organizations who saw them as too moderate and denied them their party's nomination.

On Sunday morning, Castle, 71, hustled from one community event to another, shaking hands at a fundraising walk to fight heart disease, a Puerto Rican community parade and an arts festival at a downtown park in Wilmington. After 44 years in politics, including stints in the state legislature and as governor as well as nine terms in Congress, Castle is well-known and well-liked.

But he has drawn much of his support over the years from independents and Democrats who can't vote in Tuesday's Republican primary. Ensuring high Republican turnout, his allies assert, is the key to a Castle win, and it is not at all assured.

"You belong in that Senate seat!" cheered Lorelei Hunt, a New Castle resident who greeted Castle at the Brandywine Festival of the Arts in Wilmington on Sunday. But Hunt is a registered Democrat, and she can't vote Tuesday. Her mother, Marilyn, said: "I'm going to register so I can vote" - until her daughter reminded her that it was too late to do so. "I guess I didn't realize O'Donnell was going to be a real threat," Marilyn Hunt said.