C. Rosenberg,
I'm sorry, this is confused. Proportional representation may well have added diversity, but nothing you said makes me believe that it will reduce the power of the political party. Let's get down to cases. I live in New Jersey. In national elections I vote for a congressional representative in my district (representing, what is it now, 400,000 people 500,000?) and two senators. Senatorial elections would be essentially unaffected by proportional representation since there are only two elected per state. House elections would then have to be multi-district in order for a slate to work. That diminishes regional representation (Rhode Island might not be able to contain an entire slate), but let's leave that aside for a minute. What's to stop moneyed interests from simply flooding the slate and making the field more like the field in primaries? Every hack is going to run anyway. The major parties have at least to answer to the local leadership. Once a candidate knows he's not going to get the nod of his party, he simply falls into the hands of whoever can spend enough to get his message out for him. Such campaign reform as would limit speech is not acceptable in America. Therefore, you are talking about spending reform and a slate system specifically undermines spending reform. It encourages a major party hack to use the party for all he can and then jump ship to get on the ballot, thereby effectively double-dipping. Proportional representation for ballots meaning to elect congresspeople seems to me a certain way to perpetuate and increase the influence of parties and money.
The only way I can see proportional representation working is for, perhaps, the Senate to become a purely national body, not representing the states at all. The senate, thereby, would simply become a hundred-member body chosen by a huge slate. It would make a certain amount of sense, especially in America, to have a body that is really a national legislature with no regional affiliation.
peace