M Ted, perhaps the reason the Republicans got only 10.6% of the vote in Pelosi's district in 2006 was that Nancy was considered so unbeatable that it would not be cost effective for the Republican party to put forward and support a really strong candidate in that race.    But if polls showed that Pelosi and Sheehan were virtually splitting the Democratic/liberal vote, the picture could change.   If the Republicans picked a rather fiscally conservative (balanced-budget, etc) but socially more moderate or liberal and environmentally concerned candidate -- a la the Governator -- they well might pick up enough independent voters to win the seat.    If Pelosi got 80% last time and there were 10% who voted Libertarian, Green, Independent, etc., maybe a Sheehan v. Pelosi battle would yield the two of them about 43% of the vote apiece -- IF the Republicans had a weak candidate or ineffective campaign.   But an attractive "moderate" Republican with full financial backing of the party (not to mention Karl Rove's dirty tricks like caging, robo-calls, voter challenges at the polls) would have to pull only about 12% from Pelosi and 12% from Sheehan to win with a plurality.

LittleHawk, you're right that we need either IRV or a proportional-representation system for Congress. Since the latter would take a Constitutional amendment, IRV is much more viable option.   And infiltrating the Democratic party is one avenue toward bringing that -- and other liberal policies -- about.
Transforming the Democratic party at the national level will, indeed, take time. But doing it at the local level doesn't have to.