The part of speech that "the affluent" would be would be dependent on the usage.

• If it were a subject, "The affluent" would be doing something: The affluent are cheating their workers.
• If it were a direct object: The affluent would be having something done to them: The affluent were saddled with another tax.
• If it were an indirect object "The affluent would have been the recipient of something: A tax break was given to the affluent.
• The affluent could posses something and become the possessive in a sentence: The money that President Obama wants to redistribute from the taxes of the affluent is insufficient to satisfy the deficit.
• If you want to go all Latin on the issue, you could implicate "The affluent" in the sentence and come up with: By the means of the affluent, the Democrat party realized that President Obama is a one term president.