Look, this country has a First Amendment and Miller (and others) are clearly entitled to do as they please. What's more, politics ain't beanbag, as the old saying goes, and there is nothing wrong with subjecting any candidate's record to close scrutiny during the primary process.
But it strikes me as one thing to criticize a candidate's record in contrast to another (more favored) candidate -- and simply criticizing a candidate's record without supporting anyone else. The former is part of a constructive process (supporting a preferred candidate); the latter is simply destructive, especially this early in the process.
Nor does it particularly make sense. Miller may not like all Romney's positions, but one presumes he likes them better than Barack Obama's. So why not spend one's time and money opposing the President, rather than a fellow Republican?
In addition, as much as Miller doesn't like Romney, is it possible that in some circumstance he might prefer him to, say, Jon Huntsman or Newt Gingrich or some other candidate in the field? At this early point in the process, can he be SURE?
Finally, as a prudential matter, this isn't even smart politics. If Republicans are serious about unseating President Obama -- and given the state of this country, we'd better be -- exacerbating intra-party divisions (Tea Partiers vs. Republicans, moderates vs. conservatives, etc., etc.) badly hurts that effort. Once a nominee is chosen, s/he is going to have to unify the entire party (and have some appeal to swing voters) in order to win.
What Miller is doing makes that process more difficult for whomever the ultimate nominee is. It personalizes policy differences, which creates bitterness and resentment. And it tears down, without offering any constructive alternative.
If Miller is doing this for any reason nobler than simply keeping himself "relevant" in political circles, I hope he will reconsider.