"One candidate voted against the Bush tax cuts, both times," the narrator states, among other charges.
A photo of Clinton slides over to reveal Sen. John McCain; the narrator says, "Hillary Clinton? No. John McCain."
"John McCain. Surprisingly liberal," the ad concludes.
The ad is correct that McCain voted against tax cuts proposed by the Bush administration two times, in 2001 and 2003. McCain was on the losing end of those votes; the tax cuts passed. In 2006, McCain changed position and voted in favor of extending the tax cuts. McCain has explained his position by saying that although he opposed the cuts when first proposed because of budget issues, rescinding the tax cuts after they were in effect would be the equivalent of a tax increase.
The ad gets it right that McCain voted against the tax cuts in 2001 and 2003. But by using the word "both," it implies there were only two votes. It doesn't mention the 2006 vote when McCain changed position and voted in favor of a tax cut extension. (Clinton voted in favor of the 2006 tax cuts as well.)
We have to deduct a little bit for leaving the impression that there were only two votes on the tax cuts, but the ad is correct that McCain voted against the measures. So we find the statement that McCain voted against the Bush tax cuts "both times" to be Mostly True.