I've heard this argument before and it always drives me crazy. It comes from a mindset that assumes everyone votes based on selfish personal interest. "I don't have any money, so I should vote for the people who will take away rich people's money and give it to me". You're calling people chumps for voting according to what they think is in the best
general interest instead of their own personal interest. If they don't have personal wealth and are still conservative, I call that being a person of conviction, not being a chump.
Would you have the same

reaction to a rich person that voted liberal? No, you would say that they can see beyond themselves to the greater good. What's the difference?
People
do vote out of personal interest! Hahaha, FM, I thought
I was supposed to be the idealistic one!
Unless somehow it can be argued that it's in the best interest for
everyone to vote for tax breaks for the wealthy?

People totally vote for their own best interests when it comes to initiatives and laws when it actually affects them. When it doesn't, I'm sure people are more generous.
I don't even think that's a pessimistic view of people, to me it's just politics. And btw, while I never worked directly for an elected official, I have been trained in running campaigns, involved with lobbying efforts/NGOs/PACs, and just exposed to people/groups doing 'poltical work' (I mean, I lived in DC, it's hard to avoid) and consider myself politically active myself.
So partly I have a very hard time divorcing those engaged with the political process with 'average voters' (?) I mean...have you seen people who campaigned for Kerry in the last election? Some of them practically had break-downs when he lost. They worked so hard campaigning in city after city they forgot holidays and their own birthdays. Yes they did it for their ideals -- but they banked those ideals on a particular candidate and his party/platform and they were personally extremely invested. So you could argue by affiliating themselves directly with the campaign, they were working out of self-interest...?
I dunno, maybe I've just gotten used to rabid political interest/involvement (in DC it is illegal to discriminate against someone based on party affiliation/political preference! hahahaha)
Anywhoo, for me self-interest encompasses principles, attitudes, and group identification --
group self-interest.
That's how allegiances (psychic, social, and formal) are formed and it's what partisan political systems
are built on. In many states you can't even vote outside your party as you have to register as Democrat or Republican (or Independent) period. It's assumed you'll stick that way till death. When it comes down to elections, people 97.89% vote along party lines, not necessarily along issues. Because it's assumed that your party (group self-interest) shares enough of your values/principles/views that it's a solid bet.
Constituents/voters identify with others similar to them and choose the party/candidate that that has their self-interest as a priority. And with the party system, if you don't support your group/party, then there's a good chance there'll be no one in office who supports your principles/views/ideals.
Generally speaking, the wealthy
who identify with big business will vote pro-big business, anti-union, pro-tax cuts for wealthy.
And the wealthy
who identify with hippies will vote pro-hippie.
Differences in voting pattersn amongst groups generally can be explained by looking at sub group identifications. Not all wealthy people have the same allegiances or belong to the same pyschodemographic groups, so they won't vote the same.
Maybe I interpret broadly, but generally what we assume to be 'best for everyone else' is based on our own self-interest and own particular POV.
Are you saying you think most people vote issue to issue irregardless of party identification and will vote opposite of their personal feelings? I guess maybe I'm not understanding your definition of 'self-interest'