09/13/2010 08:02 am ET Updated May 25, 2011

New Poll Shows O'Donnell-Castle Race 'Too Close To Call'

Of the weekend's new polls, the most talked about involves not next month's general election, but rather this Tuesday's Republican Senate primary in Delaware. The new survey from automated pollster Public Policy Polling (PPP) shows "a real possibility of a major upset," with Tea Party conservative Christine O'Donnell holding a three-point advantage (47% to 44%) over Congressman Mike Castle that falls within the poll's margin of error. PPP says the race is now "too close to call."

That result is stunning because Castle, a moderate Republican and popular former Governor, has led likely Democratic nominee Chris Coons by double-digit margins all year. His entry into the race last year was seen as a boon for the Republicans because Castle, as the Washington Post's Chris Cillizza reported, "is widely regarded as the only GOP candidate who can win the seat" in next month's general election.

While polling on an O'Donnell-Coons match-up has been relatively sparse, an August survey by Daily Kos and PPP and a early September Rasmussen poll both show the Democrat leading O'Donnell by margins of 7 and 11 points respectively.

Even though O'Donnell was the Republican Senate nominee in 2008, most handicappers gave her little chance against Castle. Yet aided by at least $250,000 in Tea Party Express television advertising and endorsements from Sarah Palin, Republican Sen. Jim DeMint, and the National Rifle Association, her campaign has made a significant dent in Castle's popularity among Republicans. PPP reports that Castle's favorable rating among Delaware Republicans has fallen from 60% favorable, to 25% unfavorable a month ago, to net negative (43% favorable, 47% unfavorable) now.

The only other public poll on the primary was an internal Tea Party Express survey of 300 Delaware Republicans shared with Hotline OnCall that showed Castle leading two weeks ago by six percentage points (44% to 38%).

PPP's new survey also measured general election preferences, which they promise to release later this week, but hint that Democrat Coon has benefitted from the contested primary: "He would start out with a large advantage over O'Donnell in a general election match up," writes PPP's Tom Jensen, "and is polling closer to Castle than he was when PPP polled Delaware last month."

The weekend's other notable Senate poll, conducted by the Las Vegas Review Journal and Mason-Dixon Polling & Research, shows Democrat Harry Reid with the same 2 percentage point margin over Republican Sharon Angle (46% to 44%) as they found two weeks ago. While Reid's margin fall's within the margin of error of both surveys, nine of eleven polls released since July have shown him with similar single digit margins. Reid's very narrow advantage is likely real, at least for now.