The problem with your effort is, that there are so many sources from which the political parties can get your names. You can add to this that while you can register with direct marketing "Do Not Call" and Do Not Mail lists and such, the laws governing these sources very neatly exempted public office from solicitations for funds. This has resulted in political parities and causes being one of the largest direct mail marketers in the nation.
In a moment, I am going to refer you to a web site that lists every agency that sells your name and address to direct marketers, including some used by political parties.
First, I noticed that you dismissed out of hand, the suggestion to mark "DECEASED" on your mail and return it. This was actual a very good ploy for several reasons.
The other problem with trying to use the post office to eliminate your junk mail, is the post office it self sells your name and address to political organizations, and shares its "Change of Address" system with government agencies and official semi-government and public offices like: (a) the IRS, (b) the national headquarters for the Republican Party, (c) The social Security administration, etc.
So your next attach needs to be to change your address by purchasing a mail boxes are us box or a regular U.S. Postal box, and do not use a change of address card.
You can write the agency that handles the postal service's change of address system to ask them to remove your name from the list.
Your Drivers licenses today, are used for voter registration in many states. As you update your driver's license, you are asked to register to vote. Both the voter registration card gives all registered political parties access to your address and contact information. In addition, the DMV sells your address and contact information to third party vendors.
the only solution for these political organizations, is to call them individually and ask them to take you off the list, followed up with a post card with the same information. If you request it, they have to comply.
You need to write a letter to the DMV in order to have them stop providing your contact information to third parties.
NOTE: recent fair credit reporting and consumer protection laws, limit which contact information can be given, but your name, address, and phone number are still fair game.
The sources I have just mentioned are the chief source for political parties and causes to get your name and contact information. Unlike the do not call list, there is no National Do not Mail list.
If you have a magazine on hunting Gard out doors, you may get a political mailing from the Green Peace organization, or Save the Animals, etc. The only way you can attach the political causes is to also contact the magazines you subscribe to, and make sure you revoke the right for them to include you name on lists sold to third parties.
Like wise on the INTERNET. You need to make sure you block the emails, but wait until you have respond to at least one of each organizations email, where it says, remove me from the list.
Here is the site for dealing with all th organizations who sell your name. This will take you some time, but it will be worth it.
In the future, you need to make sure you do not use change of address cards, and with each and every enrollment, driver's license renewal, bank loan, etc, make sure you do not give permission to sell your contact information. http://www.motherearthnews.com/Homesteading_and_Self_Reliance/1994_August_September/Stop_Junk_Mail_Forever
I would also like for you to take a look at this web site:
You can file a complaint for everyone that sends you junk mail after you have requested that they stop.
This is not going to be easy, it is a battle. But you can win it.
Remember, a war is not won on one front or Battle, it is won by the accumulation of wins on all fronts.
Make sure you contact all the agencies in the list on the web site I gave you.
Make sure you contact all the agencies who are sending you junk mail. (A postal card will do).
Mark deceased on all your junk mail and return it.