The acting in this movie is fine. The problem lies in the script. Near the end it gives Lee Hart, and even Donna Rice, scenes that allow us to get to know them somewhat, see what's inside them. We never get that for Gary Hart, who is far and away the most important character in this movie. We never see what made Hart so popular, especially with younger voters. We never get to see him explain important issues to the masses, though we are told that he does that very effectively. We never get scenes with him in which he gives us a hint of why he risks his career with his extra-marital affairs. He comes off as very cold, very distant, and that's problematic for a central character. As a result, we have no reason to feel anything when his career is finally destroyed.
To an extent, this is about the media's intrusion into the privacy of public officials, but that isn't examined. Nor is there any attempt to suggest a change over time to today, when a sitting president can boast about extra-marital affairs and not suffer any loss of popularity.
In the end, I was left wondering why this story was being told in 2018. It doesn't make us understand Hart, or feel sorry for him. It doesn't tell us anything either about 1988 or our own era. It doesn't make Hart a character we can feel for when he falls, because it never shows him to us as a great if flawed man. (Several characters tell us he is great, but that's not the same thing.) What was the point of filming it?