"Good intentions will always be pleaded for every assumption of authority. It is hardly too strong to say that the Constitution was made to guard the people against the dangers of good intentions. There are men in all ages who mean to govern well, but they mean to govern. They promise to be good masters, but they mean to be masters." – Daniel Webster

Of course I'd point out that the very structure of the government, including the constitution is that kind of plea...but still it underscores the flawed premise behind reformist liberalism. A politician's first allegiance is always to his own power over.