How off-base is the idea of functional voting? Form a bloc that pledges to vote for the party that will do something about their key issues, ignoring the normal

How off-base is the idea of functional voting? Form a bloc that pledges to vote for the party that will do something about their key issues, ignoring the normal party split. If their bloc has a substantial impact on the outcome, next election they have a touch more ability to influence the incumbents. So form an explicitly disloyal "party" from within the existing system. If it catches on...