The logic is syllogistic in its simplicity and conclusiveness:
Parties grow by others joining.
Your "Enemies' don't join you.
Therefore, stop treating opponents as "Enemies".
... Duh !
This is not Pollyanna thinking -- It is Strategic!
Please consider my reasoning.
People bemoan the current "extreme polemics" of American politics -- Reds versus Blues -- each side looking for a slimmest possible majority, so it can slam-dunk legislation to impose its own uncompromising world views.
IMHO, neither the Democratic nor the Republican Party will ever again grow large enough to win enough elections to for very long occupy the White House, the Congress, install compatible Justices on the Supreme Court and throughout the judical system -- And cetainly Not IF we each continue to treat our opponents as "Enemies".
It is WE (our party, which ever that may be) who put THEM at that opposing pole, by not engaging them frankly and respectfully while defending and promoting our core personal values, even when they are acting in ways that do not "deserve" our respect. The concepts of "deserve" (good or ill), "should", "must" and even "compliment" and "reward" are fundamentally coercive, and thus ineffectual methods of persuasion.
Much worse, is when we make "enemies" of those who are not really very far from us along the political spectrum.
All that "enemy" making just triggers their individual and collective immune response. No one responds well to being told that they are "evil".
In the words of Korean Great Zen Master (Dae Soen Sa) Seung Sahn
http://en.wikipedia.org/wiki/Seung_Sahn
"You make, You have." short for "You make problem, You have problem."
In this case: You make enemy, You have enemy.
Not until you think of them as your enemy are they really your enemy.
They are just people with differnet ideas -- maybe different ethics -- but not your own personal "enemy" unless you want to ban them and everyone like them Forever from your circle -- Enemies don't join. Where else will new joiners come from? Duh!
I am not arguing for the moral equivalence of all political views. I am arguing for the kind of human-to-human respect necessary to convince others to genuinely and voluntarily agree with us or find a livable compromise until we do find where we can agree.
And along the way, our opponents, treated not as our enemies, will let us get close enough to find more common ground on the really big issues where slam dunk majority legislation of uncompromising world views just makes matters worse for every one, immediately and in the long run.
A good book on this subject is Non-Violent Communication (2005) by Marshall Rosenberg.