Over the last few days, rumors that Rep. Mike Castle, R-Del., will mount a write-in campaign for the open Delaware U.S. Senate seat have gained some steam. If the rumors are true, Castle will join Sen. Lisa Murkowsi, R-Alaska, as the second major write-in candidate in the 2010 midterm elections. Murkowski, like Castle, was ousted by a Tea Party-backed candidate in what would have otherwise been a seemingly easy win for Republicans.

But who would Castle’s write-in candidacy help more? The conventional wisdom is that another well-known Republican on the ballot would hurt the actual Republican candidate. But could a Castle write-in campaign snipe enough votes from the Democratic nominee, Christopher Coons, to help the Republican nominee Christine O’Donnell win?

Delaware has three counties: New Castle, Sussex, and Kent. New Castle is the most populous and tends to vote Democrat. However, it has a large number of independent voters. Sussex is the most conservative of the counties and Kent, sandwiched in the middle, is usually the most competitive.

It’s safe to assume that O’Donnell will win Sussex County. It’s also safe to assume that a Castle write-in campaign would make little in-roads here, since Castle lost the county to O’Donnell in the primary by a nearly 2-to-1 margin.

Kent County was another nearly 2-1 win for O’Donnell. In 2008, McCain won 45 percent of the votes from Kent County. In a midterm election, where turnout will be lighter than it was in 2008, Kent County could definitely be in play.

Castle would most likely make the biggest impact in New Castle County, the only county he actually won in his primary battle. Before he lost the Republican primary, Castle led Coons in head-to-head polling by double digits. Some polls showed him garnering nearly one-third of the Democratic vote.

New Castle County is where the race will be won or lost. Castle can steal enough conservative Democrat and independent votes from Coons to make the race a toss-up. A split New Castle County would make it possible for O’Donnell to win by taking Sussex County and splitting Kent County.

Castle still has not formally declared his intent to mount a write-in campaign and O’Donnell is still down by double-digits in the latest polling. But a Castle write-in campaign could change this race dramatically.

Stuart James is the Founder and Editor of www.threefingersofpolitics.com.