Millsboro, Delaware (CNN) - The Republican party's candidate in Delaware says he won't be caught off guard by the Tea Party in the final days before Tuesday's primary.
Mike Castle, the moderate Republican congressman whose Senate bid has the support of the state GOP, is trying to avoid becoming the next Lisa Murkowski.
Sen. Murkowski, the Republican incumbent in Alaska, was ousted by a Tea Party challenger last month. Soon after her defeat, she phoned Castle with a warning, the congressman tells CNN.
"I actually received a call after the election saying, 'Mike, you need to be prepared, they'll come at you hard,' " Castle said after a campaign stop in Millsboro, Delaware. "It was part of a broader wake-up call that's been going on for several months now."
So far this year, Tea Party activists have claimed insurgent victories against Republican moderates in Senate races in Alaska, Utah, Kentucky, and Nevada, with their message of more individual liberties, less government and lower taxes.
Not long ago, the national Tea Party Express organization announced their support for Castle's opponent, conservative underdog Christine O'Donnell. Two of the movement's leading lights also endorsed her in the past few days: former Alaska Gov. Sarah Palin and South Carolina Sen. Jim DeMint.
O'Donnell tells CNN, she believes Tea Party adherents are "rising up to take back the country, because they're concerned about the direction of Washington D.C."
Since Tuesday, the Tea Party Express has spent at least $150,000 on ads supporting her, federal records show; they have pledged to bring that total to $250,000 by Tuesday.
Castle says that effort has been enough to make him "sit up and take notice." His campaign, and the GOP, are trying to portray O'Donnell as a long-shot fringe candidate, hitting her with ads and statements questioning her inexperience, her electability, her campaign's finances and even her personal financial problems.
"It's not the way I prefer to go," said Castle. "I've not in the past ever gone in that particular direction."
Likewise, O'Donnell and the Tea Party have been knocking Castle as an "Obama Republican" who voted in favor of the 2008 bailout and Democratic bills on energy and campaign finance, and portraying him as a party establishment candidate.
"This is a year where, in order to get things right back in Washington, we have to replace career politicians with citizen politicians," O'Donnell told CNN as she campaigned at a senior center in Dover.
But Castle still has many advantages in his bid: a well-known name, the support of state party leaders, a record of years of public service and a lopsided cash advantage.
According to Sam Hoff at Delaware State University, he is one of the most liberal Republicans in the House, when scored by voting records.
But Jason Mycoff at the University of Delaware says, "even among primary voters, he's a decent match. The Delaware Republican Party is not like Republicans in the South. They're not really right-leaning."
By contrast, if it is O'Donnell who wins the nomination, Hoff says she would be in for an uphill battle in November against Democrat Chris Coons. "She would face a Democratic state, a blue state," he said. "Obama's approval level is over 50 in Delaware, compared to mid-40s nationally."
But still, he says, in politics, "never say never."
Whichever candidate wins the Republican nomination, the fight may leave both with some bruises – which could help Democrats cling to the seat. It was held by Joe Biden for decades, and is now temporarily filled by an appointed Democrat.
"This back-and-forth is probably not positive for either her, or me," Castle told CNN.
But Diamond State Tea Party organizer Kevin Street says he doesn't think a rough-and-tumble primary is going to hurt the GOP.
"I think stronger debate brings stronger candidates," he said. "And if it hurts us, you can't blame ourselves for trying."
–CNN's Brian Todd contributed to this report