I believe that 33% of Americans think Obama is the worst president. I certainly don't believe that statistic is worth the sensationalist headlines. Isn't that only a little bit higher than the percentage of birthers?
Also, as already pointed out, Dubya and Nixon had to compete for Democratic/left-leaning votes whereas the right wingers had an easy choice.
Nixon was President a long time ago. Anyone under 40 wasn't born or doesn't have many memories of the Nixon years.
And if 30-40 year old presidencies are fair game, Obama had to compete with Jimmy Carter for Republican/right-leaning votes. Carter is still is one of the worst, least effective, jokes of a President since World War II. He had weak foreign policy during a time of high gas prices, high unemployment, an alien invasion and general malaise. Sound familiar?
People aren't voting for Nixon because they have many memories of the Nixon years but because they have one big one: him resigning in disgrace. The effect that event has on the W vote in this poll isn't comparable to the effect on the Obama vote of Carter, a 90-year old man people like as a person if they can even remember his name. (Most born during his presidency or after probably don't.) Even after 6 years with Obama as incumbent, more people are still voting for a Republican as worst president than a Democrat.
I'd argue Obama is one of the best on the most important stuff: pulling the economy back from the brink of the biggest crisis since the Great Depression, biggest reduction in # of Americans with no health insurance since LBJ, muscling through new power plant emissions rules, presiding over operation to kill Bin Laden, navigating the expiration of Bush tax cuts on richest through a recalcitrant GOP House, presiding over the deficit being cut in half as promised, off-setting the damage Reagan and the Bushes did to the Supreme Court, staring down the GOP (and the media) on the debt ceiling, effectively forcing them to give it up as a weapon. And hardly any legitimate scandals in a climate where the GOP and the media desperately try to convince people otherwise.
