That depends. If someone deliberately, grossly, and unapologetically tries to mislead you, is he a liar?
Essentially all politicians, and most people, will lie and mislead at least to some extent. Most, when caught, will apologize and correct. For example, in a Wall Street Journal article trying to pooh-pooh forces under Obama's command killing bin Laden in a gutsy, on-the-ground raid, Karl Rove quoted Bill Clinton as saying "I would have done the same." What Clinton actually said was "I hope I would have done the same," which completely changes the meaning. When this was pointed out, the Wall Street Journal printed a retraction and corrected the piece on their web site. You see, the Wall Street Journal believes in telling the truth.
Romney, apparently, does not. I will limit myself to two examples.
1. In one of the Republican debates, Newt Gingrich complained about a misleading attack ad directed against him. Romney defended himself by saying he hadn't seen it, suggesting that the ad must have been from a SuperPac not under his control. Later in the debate, the moderator revealed that the ad ended with Romney saying he approved it -- the ad was from the Romney campaign after all! It is possible that the campaign put the endorsement on without Romney's knowledge, but Romney did not apologize, did not correct, did not investigate, and no one at the campaign was punished for misrepresenting the boss.
2. In Romney's very first TV ad there is a segment where you hear Obama's voice saying various things. The last bit goes "If we talk about the economy, we lose." This seemed odd to me because Obama talks about the economy all the time. It turns out Obama said that in 2008 and was he quoting the John McCain campaign when the rapidly declining economy was a huge minus for Republicans. When this was pointed out Romney said (I'm paraphrasing) "We hit him, and we're going to keep hitting him!" In other words, grossly, deliberately and unapologetically misleading you is a good thing and he intends to keep doing it.
You can win elections without the truth, but you can't govern America well without it. Remember that this November, and remember how Romney is treating you now.
Thanks to Rachel Maddow for pointing out the Karl Rove angle and for spreading this meme far and wide.