There's quite a difference IMO between a strategy to unify liberals and conservatives or alternatively to position ourselves as centrists without apology.
The latter strategy can quickly capture the 35-45% voters who are themselves centrists. Then fairly slow progress may succeed in getting close to or above 50%.
But I'm beginning to think you may the former strategy in mind. The extremes are 55-65% of voters, to compete for them and have some modest success would be impressive and would influence politics for the rest of our careers.
The unifying strategy is very hard work and requires a lot of energy and money, also excitement.
The centrist strategy is really very easy, except that its so foreign to professionals, and maybe distasteful. You just have to claim the center and defend it. Its so easy a professional wrestler can do it. It would not take much money or energy. It could be done very quietly I think.
I will quickly mention what I disagree with, then move on to what I agree with in what you said. I do not believe that the extremes are represented by 55%-65% of the population. I think that percentage is good for those who are extreme or heavily lean towards one party, but of that group, I think half could be peeled away if given an alternative. BTW, I think your 40% figure for true independents is spot on. Given that, 40% alone wins over 60% that is divided by two, right?
That leads me into the part I totally agree with you on in your post. Trying to convert ideologues is a waste of time. As this group has clearly demonstrated since they run each party that compromise is not in their vocabulary, hence very little is done as problems continue to pile up. Therefore, attracting the centrists and those who are constructive, or like seeing something done towards solving problems has the greater potential.
I think the main flaw with the extreme elements is that being locked into hardened philosophical beliefs creates a situation in which each group shows up with a half-empty toolbox. One party believes government is the solution to everything while the other party believes business can solve every woe. Personally, I think both are equally wrong and foolish because each entity has strengths and weaknesses. For me, belonging to a party that has a full toolbox ready to go to work is attractive. If such a party existed it would have a philosophical belief of identifying what is a problem, eagerly accept ideas from all sources, toss the ideas around to come up with what the majority believes is the best solution, and accept the outcome even if you didn't get your way so that the group can move onto the next problem. Once a problem has been identified, doing nothing is not an option and obstructionists are removed.
I think something like that would be attractive to a lot of people, but not the ideologues.