Even as the tea party movement vows to spend $250,000 in the coming days to defeat Rep. Mike Castle, campaign officials from both sides of the aisle concede that Castle should capture the Republican Senate nomination in next week’s Delaware primary.

Signaling that the threat from tea party favorite and self-described marketing and media consultant Christine O’Donnell is believed to be exaggerated, the National Republican Senatorial Committee does not plan to devote any resources to help Castle survive the Sept. 14 primary. National conservative groups have been reluctant to support O’Donnell. And Washington Democrats this week have stepped up their offensive against Castle, already looking ahead to the general election contest against the Democratic nominee, County Executive Chris Coons.

“I think that her winning is probably not likely,” one Democratic campaign aide said of O’Donnell. “Castle losing is the exception, not the rule. But [the internal fighting] still muddies this for him.”

Sen. Jim DeMint’s (R-S.C.) Senate Conservatives Fund, which has helped boost the candidacies of several nonestablishment Senate candidates this cycle, has shied away from endorsing O’Donnell, largely because of her questionable viability in the general election. But DeMint has signaled that he supports her policies. Former Alaska Gov. Sarah Palin (R) has been silent on the Delaware race as well. And citing the same viability fears, the Wall Street Journal urged voters to back Castle in a Wednesday editorial.

“She has little chance to win in November,” the editorial said of O’Donnell. “A two-time loser statewide, Ms. O’Donnell has a history of financial troubles and recently told the Weekly Standard her home and office were vandalized, though she hadn’t reported it to police. She recently accused a conservative local talk radio host that he had been ‘paid off’ by Mr. Castle’s supporters after he asked her tough questions.”

Polling released Tuesday reaffirms the viability questions.

A Rasmussen Reports survey taken Sept. 2 found that favorables for Castle, a two-term former governor and Congressman since 1993, were at 66 percent compared with O’Donnell’s 39 percent. And like the Wall Street Journal editorial board, Republican campaign officials quietly acknowledge that an O’Donnell victory would all but ensure that Republicans lose their tightening grip on the seat held by Vice President Joseph Biden for more than three decades.

Rasmussen’s poll of 500 likely voters gave Castle an 11-point edge over Coons. O’Donnell, however, trails Coons by 11 points.

But the numbers have not dissuaded the Tea Party Express, one of the few organized arms of the anti-establishment movement, which was emboldened by Joe Miller’s unexpected win over Alaska Sen. Lisa Murkowski in last month’s GOP primary. The organization has now turned its full attention to Delaware.

O’Donnell is one of three candidates featured in a fundraising appeal on the front page of the group’s Web site. The other two are Miller and Nevada’s Sharron Angle.

Tea partiers hosted a rally for O’Donnell on Tuesday and are expected to fuel a media blitz in the coming days.

The tea party’s success in Alaska served as a wake-up call of sorts for both the Castle campaign and the Delaware Republican Party, which has aggressively moved to counter O’Donnell’s “vulgar attacks” in recent days following months of silence.

“This is a candidate who not only spins different elements of different stories, but also flat-out lies,” Tom Doheny, spokesman for the state Republican Party, said Tuesday.

The state GOP’s list of complaints against O’Donnell is long and unusual, topped by her former staffers’ accusation that the 71-year-old Castle had engaged in a gay love affair.

“What shocks us is that a fellow Republican has come out so vulgarly against another Republican,” Doheny said. “We just don’t operate that way around here.”

A Castle campaign aide suggested that the worst may be yet to come.

O’Donnell “claims to have this big video that she’s going to unleash in the last few days of the campaign that shows Castle taking cash from Joe Biden. We’re kind of waiting for that to unfold,” the aide said with an incredulous laugh. “She says that it’s going to be earth shattering.”

Formally, the campaign offered this statement: “Voters in Delaware have come to expect O’Donnell’s misrepresentations of the Congressman’s record, the personal attacks on his family, and wild accusations,” campaign manager Mike Quaranta said. “We will stand in strong defense against her false allegations and disgusting tactics with a series of TV and radio ads to highlight the reasons why voters across Delaware trust Mike Castle’s record of fiscal responsibility and to identify factual information about Christine, as reported in local media sources.”

But Castle has been pushed to go negative as well in recent days.

His criticisms of O’Donnell, which largely mirror those of the state GOP and some national conservative voices, are outlined in a 30-second attack ad that began airing this week.

“Do you recognize this Delaware politician?” the narrator asks over ominous music. “Here’s some hints: According to the News Journal, she didn’t pay thousands in income taxes, had to be sued by a university for thousands in unpaid bills, defaulted on her mortgage, she ran up a huge campaign debt, and left vendors and staff unpaid while using campaign funds to pay her own rent and personal expenses.”

The O’Donnell campaign did not respond to requests for comment.

But Delaware’s primary system may offer her one advantage. Only those registered as Republicans as of March 30 are allowed to vote in Tuesday’s contest.

Voter turnout is traditionally light for primaries, according to Delaware Commissioner of Elections Elaine Manlove. About 30 percent of registered Republicans participated in Delaware’s 2008 presidential primary.

The state reports 182,796 registered Republicans — just 29 percent of all registered voters.

“We have some hot races here,” Manlove said. “Now that the tea party has come to town, we’ll see what effect that has on turnout. It’s kind of a new phenomenon for Delaware.”

Democrats, meanwhile, have decided not to wait until the primary to start lashing out at the expected winner.

The Democratic Senatorial Campaign Committee launched a new Web site, titled “Mike Castle’s Washington House of Waffles,” that blasts him for being “in the waffle business for years.”

Later in the day, the DSCC fired off a separate news release criticizing Castle for paying penalties and interest on property taxes on his Washington apartment on at least three occasions.

“Politician Mike Castle has been in Washington long enough to know that those in glass houses shouldn’t throw stones,” DSCC National Press Secretary Deirdre Murphy said. “After doing his best to slime his opponent over her failure to pay taxes on time, it turns out Castle also has a bad habit of forgetting to pay his taxes. Whether it’s his constant waffling on the issues or his hypocritical attack ads, Mike Castle is proving why he’s just another Washington politician who has been in office for far too long.”