Neither, really. I was attempting to be a bit more nuanced than that (I was already long-winded as all hell).
> Are you saying that politicians are so out of touch with reality that they don't know what they promise is impossible?
No, I think the vast majority of voters more reliably fall into this camp, actually. Politicians are quite aware that they make unfulfillable promises, knowing much of it is (likely) impossible, but they do it because it has become the expectation in American elections that politicians campaign on issues, and they have to choose issues they think will resound strongly enough with the voting public.
> Or are they being intentionally misleading?
Well, to a certain extent--that extent being to the full degree to which they make unfulfillable promises--yeah, if I was being pedantic about it. I don't think they are completely misleading in that I am certain many candidates do believe strongly in certain issues and are genuinely interested in "fighting" politically for those issues.
With congresspersons, I don't think this problem is as acute as it is with presidents. They make promises that are legislative in nature, and can actually deliver (or be criticized for not delivering).
Unfortunately, campaign promises were adopted into the popular election of presidents, and I think are detrimental for voters and the presidents themselves. Presidents operate within relatively (and sometimes heavily, depending on the political climate) circumscribed spheres of influence. Yes, presidents can (and do) suggest an agenda and even outright offer legislation to Congress, and work to spend political capital to the advancement of that agenda. But, ultimately, Congress has the authority, power, and responsibility where lawmaking that affects the People is concerned. But presidents seem (from the opposite aisle) to receive a lot of unwarranted blame for congressional actions--and then the congressional actors go on to spend many more terms in office.
Where I do believe politicians are being intentionally misleading is on the campaign trail, specifically where contenders take on incumbents and criticize them for mostly legislative actions that are beyond their control, instead of for executive actions. If Mr. Romney wants to attack Mr. Obama for the ways in which he is executing the law, or representing the policies of the nation to other heads of state, etc., okay. But when he attacks the President for things that happened in Congress, it is fundamentally misleading. It leaves people confused about the structure and organization of government, effectively perpetuating the same cycle.