It's an old billboard, pretty sure it was put up during 06 midterm election. I do remember the logic behind it.
The guy who put it up argued that the 9/11 attacks were Bill Clinton's fault, and that's why people shouldn't vote for Democrats. In 1998 al-Qaeda bombed two US embassies, one in Tanzania and Kenya. President Clinton ordered that bin-Laden be either captured and tried, or killed. Later that year the US did a cruise missile attack against al-Qaeda, but as we all know bin-Laden wasn't killed in the attack, nor was he captured. Since he got away, and later planed the 9/11 attacks, it's Clinton's fault 9/11 happened since he didn't do enough to stop bin-Laden.
Of course this ignores the Bush administration overlooking a document with a title like al-Qaeda determined to strike in the US, or something similar.