Coins of the realm
(Go here for background)
It is not the goal, nor should it be the goal of single-issue groups like The Sierra Club to build a national party. It is the goal of the Sierra Club to protect the environment.
The objection I and others have with the Sierra Club's short-sightedness (and the short-sightedness of other progressive groups) is not that it hurts the goal of building a national party. My objection is that the Sierra Club is hurting their own long-term goals by focusing only on the individual records of the people they endorse.
Lincoln Chafee, Republican Senator from Rhode Island, could be getting a 100% environmental rating from The Sierra Club, but if he continues to prop up a party that has a 0% rating, then supporting him is dangerously counter-productive.
Markos uses the NRA as a counter-example. The NRA doesn't endorse individuals as individuals. They endorse as part of a strategic plan designed to protect their interest. They don't endorse candidates just because they have a 100% rating. They endorse them if doing so will improve the chances of the NRA getting positive results in the next legislative session.
Endorsements are one of the coins of the realm in politics. Progressive single-issue groups have been spending them unwisely on feel-good candidates instead of investing them wisely in strategic plans with a promise of longer term dividends.