As Rep. Mike Castle (R) ponders a write-in campaign for Delaware’s open Senate seat, it became clear Monday that there’s more at stake than his political legacy.

A Rasmussen Reports poll found that a Castle write-in bid could breathe new life into the candidacy of GOP nominee Christine O’Donnell, the tea-party-backed conservative pundit who has been widely dismissed as too conservative to win the general election

According to the poll, a Castle write-in candidacy would derive its support most directly from supporters of Democratic hopeful Chris Coons. The poll found Castle would cut into Coons’ support by at least 5 percent, potentially more, turning a likely blowout for Democrats into a more competitive contest.

The survey of 500 likely Delaware voters on Sunday gave Coons 49 percent, compared with 40 percent for O’Donnell and 5 percent for a Castle write-in, according to Rasmussen, which became the first polling firm to survey the effect of a Castle write-in campaign. The 9-point difference between Coons and O’Donnell is a far cry from the 15-point or larger disadvantage reported by three separate public polls earlier in the month.

‘Castle has not indicated that he will run a write-in campaign and it is likely that his support could increase if he were to do so,’ the Rasmussen findings noted. ‘Rasmussen Reports did ask Castle supporters who they would vote for in a two-person race and virtually all said either Coons or not sure.’

Castle has until Thursday afternoon to make a decision.

‘You’re talking about a remote possibility at this point,’ he told Delaware’s News Journal in a front-page article that appeared Monday.

The O’Donnell campaign largely ignored the Castle effect outlined in Monday’s survey, seizing on the narrowing gap.

‘The Rasmussen numbers are encouraging and represent another sign that Delawareans are eager for a candidate who shares their priorities of a pro-jobs economy, lower taxes and responsible spending restraint,’ O’Donnell said in a statement. ‘As more people find out about my opponent’s rubber stamp approach to the Obama/Reid agenda, which is bankrupting the country and killing jobs, our momentum will grow even more.’

Regardless of whether Castle reinserts himself into the race, O’Donnell is fighting to prove wrong the political establishment, most of which has written her off.

Having raised nearly $2.6 million in the two weeks since her unlikely primary victory, O’Donnell hired a new press secretary, scheduler, attorney and the California-based Fred Davis, whose firm, Strategic Perception Inc., produces commercials for California Senate candidate Carly Fiorina.

Further, the Delaware Republican Party, once an aggressive O’Donnell adversary, has given her use of an office and a staffer to coordinate phone banking and other voter outreach efforts. And the National Republican Senatorial Committee, having already donated $42,000, is in regular contact with the campaign. But don’t expect a face-to-face meeting between O’Donnell and NRSC Chairman John Cornyn in the near future.

O’Donnell canceled a scheduled meeting earlier in the month and has yet to reschedule.

Meanwhile, Democrats on Monday sought to downplay the potential Castle effect outlined in the poll, while encouraging the former governor and longtime House Member to retire with dignity.

‘Write-in campaigns are really, really hard,’ said Katie Ellis, acting executive director for the Delaware Democratic Party. ‘It seems like a better move for his career to end it on a high note. There are a lot of Republicans and Democrats that respect him. I think he might lose a lot of that good will.’

One Delaware newspaper agreed.

‘A write-in campaign to win at any cost does not befit Castle’s selfless, apolitical reputation,’ read a weekend editorial in the largest newspaper in the southern, more conservative, part of the state. ‘Joe Lieberman, who weaseled his way back into the Senate … is forever tarnished as an overambitious egomaniac. Mike Castle is better than that. It’s time for him to retire gracefully.’

Lieberman didn’t stage a write-in campaign and instead became an Independent on the ballot, but his 2006 Connecticut Senate race may be the most recent example of what political observers say is an extremely rare occurrence in Congressional elections: a Senate race with three viable candidates.

Senate Historian Donald Ritchie said just one candidate, then-Democrat Strom Thurmond (S.C.) ran a successful write-in campaign. And that was in 1954.

Beyond the Lieberman contest, Ritchie could recall just one Senate race ‘ the 1980 defeat of four-term Sen. Jacob Javits (R-N.Y.) ‘ that involved three viable candidates. And he could not recall any year that featured more than one such contest.

This year, of course, there may be three: Alaska, Florida and Delaware, should Castle launch a write-in bid.

‘To have three would be pretty remarkable,’ Ritchie said.

Putting aside some messy races during the Southern Realignment, the few competitive three-way Senate elections before 1980 took place in New York, where election rules allow candidates to secure ballot placement with third parties relatively easily, according to political science professor Gary Jacobson of the University of California, San Diego, who has studied Congressional elections for four decades.

Those three-party elections, as was the case with Javits, usually benefited Republicans.

‘In this case, if there’s a split, it’s a split on the right, not on the left,’ Jacobson said of the Delaware race. ‘It’s going to be very interesting. You had an election where Republicans had everything going for them. Maybe too much. … If [O’Donnell] were to win a general election, that would be a shock.’