I know this is ad hominem, but Robert McDowell is one of many corporate lobbyists appointed by Bush to undermine a federal agency.
- attacking an opponent's character rather than answering his argument. http://dictionary.reference.com/browse/ad+hominem
It's also hard to see how it is relevant.
As for his funding source, which of his arguments does that invalidate? Which of his facts is false, as a result of him being funded by Verizon (or whoever)?
(By the way, I don't even agree with him. Just pointing out that ad hominem is a logical fallacy, except under fairly limited circumstances - specifically when facts are presented that the reader cannot verify.)
[Edit: just thought of another circumstance where it isn't a logical fallacy. "You are wrong because of XXX. Also you are stupid." In this case, the argument is unchanged regardless of the attack on character.]
In the context of an essay, there is no opponent. The author is responsible for drawing conclusions from evidence which they use to support their thesis.
The corollary is an opinion piece. The author is free to derive evidence which supports their pre-determined conclusion. Such articles are able to be complete fabrications and misleading while lime-lighting in our media as legitimate articles to report. If the reader is not aware of the author's bias they may be mislead in their own conclusions.
Therefore it is not ad hominem to draw into question the bias of an author of an article. Doing so may invalidate any argument they have made which relies on the intended omission of evidence which may counter their argument. Sometimes it is the only way by which a reader can distinguish an opinion piece from a proper essay when the distinction isn't made clear by the publisher of the article.