Mercury News Editorial: "I'm Barack Obama, and I approve this message." Voters are used to that tagline on political ads, and most get the idea behind the decade-old law requiring it in federal campaigns. Known as the Stand By Your Ad provision, it forces candidates to take responsibility for whatever facts or foolishness their TV commercials contain. Without it, the mudslinging might be even dirtier. Shouldn't the same principle apply to political ads made by people and groups who aren't officially associated with candidates and ballot measures, but wield just as much influence in elections?