Liberty: This is not an ad done by a campaign who thinks it is winning. It seems more designed to shore up the base. Americans do take their presidential elections seriously, and this isn't going to influence independents.
An interesting thing to note in this election is that both campaigns are heavily focused on motivating their bases to get out and vote, more so than trying to convince independent voters. Presidential elections have been trending this way, but it's even more pronounced this year. This is because politics have become so polarized in the US that the proportion of the population willing to vote for one of the major parties' candidates and not being convinced on which one to vote for is quite small. Instead, you have die-hards on both sides who are going to vote for their party's candidate regardless, people who lean strongly towards one party or the other but are disillusioned enough that they don't feel particularly motivated to vote unless something really gets them fired up, and then independents who increasingly feel that both parties are full of wankers and thus don't feel inclined to vote. Basically there are more gains to be had in just trying to get the base pissed off enough to bother to go out and vote than there is in trying to convince people that the candidate has good ideas on how to run things. Which is a pretty sad commentary on the state of affairs in US politics.