Most successful groups actually exercise influence through get-out-the-vote. That is, instead of having 60% of people who favor candidate X anyways to vote, they get 80% to vote and change the minds of 5% of people favoring candidate Y, within the single-issue group. The result is still a large impact; the NRA is very successful at this, and combined with it's legal and policy work, substantially influences national legislation.

The NRA is HUGE and well-coordinated, and by most reasonable measures has "won" it's policy battles consistently.

Targeting close districts is also an old strategy, but targeting primaries can produce higher yields since people are more amenable to switching votes between candidates within their favored party than to abandoning their party.