If I asked you to describe the things you dislike about a prominent politician from the other party, you could surely come up with a long list, and "I disagree with him on issues" would be only one. You'd doubtless be able to describe a series of character flaws and disturbing tendencies that could in theory could apply to even members of your own party. But certain traits that we sometimes associate with politicians generally—pathological ambition, dishonesty, ruthlessness—we almost always ascribe to the those in the other party, while forgiving them in those who seek the same goals we do.
To a degree, that's natural and almost everyone does it. But it becomes analytically problematic when you convince yourself that everything a particular politician does or says is a lie, nothing they say can be taken at face value, and their every motivation is dark and sinister. For instance, here's something Charles Krauthammer, who gets more admiration for his intelligence and insight from his conservative brethren than any other pundit, said about Hillary Clinton:
"Does anybody imagine that Hillary burns for anything, for the middle class, for gay rights, or anything of that sort? The Clintons have been motivated throughout their lives by by the wish to rise and to acquire power, that's who they are, and that's why she's stumbling over herself on the wealth issue…Who believes that she really feels it [concern for the poor and middle class] in her soul the way that you would have felt it with a Bobby Kennedy or FDR?"
Krauthammer doesn't offer any evidence for Clinton's supposed lack of sincerity on this issue. He just knows it's true, because it's Hillary Clinton, right?
Sometimes politicians change their positions on issues for momentary political advantage, and you can surmise that their new position is insincere, but the Clintons (he was talking about both of them) have advocated for the economic interests of ordinary people their whole careers. There are plenty of critiques one can offer of their records on that score, but it's an odd thing to assert that it was all a big con, a cloak of concern they donned just because they thought it would be politically advantageous.
Now maybe Krauthammer is just playing a role here, that of the partisan fist-shaker whose job it is to nurture and sustain the Fox audience's burning hatred for Clinton. But why can't she both be ambitious and actually care about regular people? The two aren't incompatible; it isn't as though if you have political ambition then you can have no true beliefs about policy. You know who else had a lot of ambition? Every politician Charles Krauthammer ever liked. That's because every politician has a lot of ambition. If you're going to shake a million hands and give a thousand speeches and beg people for money and work your way up from city council to the Senate or the White House, you need lots and lots of ambition.
This is a common criticism we hear of people running for president, that they have a burning desire to achieve the office but the reasons they give for their candidacies are a lie, because nothing matters to them but power. They share part of the blame, since the answer they give to "Why do you want to be president?" is usually a bunch of piffle like "I want to restore the American Dream" or "I just care so damn much about this country and I can't bear to see it go on the path its on now." But the implication of Krauthammer's argument is that what we ought to seek out in candidates is ideological crusaders, which is fine if you're on board with the crusade, but tends to be horrifying if you aren't.
While there have been presidents who were forced by circumstances to break a promise or abandon a goal, there has never been one who said the oath of office, then turned to the crowd and said, "Ha ha, I've fooled you all!", then proceeded to follow an entirely different agenda than the one he campaigned on. Lots of politicians lie, and all politicians spin. But you'll always do better to assume that in most cases and in broad terms, they believe in the things they advocate for.