Warren Buffett spent an unpleasant part of last weekend's Berkshire Hathaway (NYSE: BRK.A) stockholder party defending the company's investment in PetroChina Co Ltd ADR (NYSE: PTR), and its support of the repressive Sudan government. While he made the reasonable argument that in China, influence runs down from the ruling committee, not up to it, the pressure to divest holdings of companies that support the Darfur genocide is sure to continue.

But which companies are these? One often quoted source for suspect companies is the Sudan Divestment Task Force, a group formed in 2005 by college students concerned about the issue. They have compiled a comprehensive list of firms they believe support in some way the Sudan government. In partnership with Invested Interests, they have used this list to develop a screening tool that allows investors to check their stocks and mutual funds for such companies.

A complication to the issue might arise as Buffett makes good on his promise to donate most of his Berkshire Hathaway stock to the Gates Foundation. Given the Foundation's strong involvement in Africa and with the poor, as it begins to accumulate BH stock I'd expect it to ask hard questions about the PetroChina holdings. Ironically, it could be from this board that renewed pressure will be brought on the company to divest itself of PetroChina.

Addendum: after reviewing SDTF's Sudan Company Report which they shared with me, I can report that no U.S. based companies appear in it. The worst offenders include Petronas of Malaysia and several Middle-east based oil production firms.

Activists should leave Warren Buffett out of the debate over the genocide in Sudan's Darfur region.

Following Buffett's suggestion, Berkshire Hathaway Inc.'s (NYSE: BRK.A) shareholders on Saturday defeated a proposal requiring the company to divest its holdings in PetroChina Co. (NYSE: PTR) because of the ties of its parent company to Sudan.

Though I share the outrage over the Darfur genocide, people who are trying to pressure Buffett are misguided. Though he is the largest shareholder in PetroChina, his influence is dwarfed by the the Chinese government, the real bosses of the company's management.

Why would the Chinese government listen to Buffett? Shouldn't the U.S. government and the UN be pressuring the Chinese to pressure the Sudanese?

Maybe Buffett could send a message by selling PetroChina's shares. But for every seller, there is a buyer. All Buffett would do is wind up making someone else rich, negating any message he was trying to send.

If you want to express your political views in your portfolio, invest in one of the many socially conscious mutual funds, including Citizens Funds or Domini Social investments. Investment News says that this category is the fastest growing segment of managed money.

But trying to separate the good companies from the bad ones isn't always easy.

"The pressure on social investing portfolios to achieve a beta comparable with those of such popular benchmarks as the Standard & Poor's 500 stock index means that social funds include companies that are involved tangentially in socially conscious activities," according to Investment News.