For months, supporters of conservative Republican Senate candidate Christine O’Donnell have been hearing the same argument from the Delaware GOP establishment: “Well, sure, she’s a good conservative, but Mike Castle can win.”

Castle’s 2007-08 ratings (20 and 28, respectively) by the American Conservative Union marked him as one of the most liberal Republicans in Congress. His establishment backers have argued that the best Republicans can hope for in Delaware is to elect a RINO whose voting record makes Charlie Crist look like a raging right-winger. As one O’Donnell supporter told me, “Castle is Delaware’s answer to Dede Scozzafava,” referring to the liberal GOP candidate who quit last year’s 23rd District special election campaign in New York and endorsed the Democrat.

Last week, the “Castle can win” argument suffered what may prove a fatal blow from a Rasmussen poll: “Congressman Mike Castle’s support has fallen below 50% for the first time in his race with Democrat Chris Coons for the U.S. Senate in Delaware.”

Castle, who had previously led Coons by more than 20 points, now leads the general election matchup 47-36, which is only marginally better than O’Donnell’s 41-39 matchup with Coons.

“This shows what we’ve been saying all along — Christine can win,” said one source close to the O’Donnell campaign.

O’Donnell has important support from conservative New Media, including endorsements from Red State’s Erick Erickson and Pamela Geller of Atlas Shrugs, and was featured last month on Mark Levin’s popular radio show. Last week, she picked up the endorsement of the pro-life Susan B. Anthony list, which may help O’Donnell close the fund-raising gap with Castle (according to the June 30 FEC report, he had more than $2.6 million cash on hand.)

With the Sept. 14 primary now less than two months away, some O’Donnell supporters are beginning to ask a question that has proven crucial in other GOP primaries: “Where’s Sarah Palin?”