Campaign finance issue or free speech issue?

I tend to say "free speech issue," but you make your own call.

In Dunn County, a website has sprouted up in opposition to the re-election campaign of Dunn County Sheriff Dennis Smith. The website, dennissmithforsheriff.com, goes after Smith hard, and it doesn't look like either the Republican or Democratic parties in Dunn County are very happy about it:

The chairs of the Dunn County Democratic and Republican parties have sent a letter to the State Elections Board requesting an investigation into a Web site that makes allegations against Sheriff Dennis Smith.

The site urges people not to vote for Smith and does not contain a disclaimer showing who is paying for it, according to a news release sent by Dunn County Democratic Chair K. Lynne Hausman and Republican Chair Maripat Krueger.

My impression of the site is that it is an individual, likely a connected individual, who is exercising his or her free speech rights. I cannot speak to the accuracy of the claims, though. In fact, I don't even know what party Smith belongs to. What do you think? Does the government have the right to punish the proprietor of the anti-Smith site for not making a disclosure of who paid for it?