Because, in today's America it could get you a visit from your local sheriff and the FBI.
The Left / Mainstream Media like to remind us, back when they were fighting "The Establishment" in the 1960's, that the actions of the government toward them - police raids, FBI wiretapping, etc. - were tyrannical, antithetical to freedom and liberty, and jack-booted stomping all over the Constitution. But now that they are The Establishment, they've gone all Nixonian, drawn up enemies lists, and justified harassing the opposition on national security grounds. (I wonder, is the case of Mr. Bowler of Greene County, Missouri, an isolated incident? Hmmm... -ed.)
I agree that public officials - indeed, all Citizens - need protection from those who want to do them physical harm. And if someone is a credible threat under The Law, then yes, they should get a visit. But calling your elected representatives to account for their actions (or, inactions) and harming their chances at the polls by dissent, does not rise to that level. Instead, it is the Right of every Citizen (See Amendment, First) in a free society.
If dissent was patriotic in 1971 (Vietnam, Nixon), 1981 (Reagan, Cruise Missiles), 1991 (Gulf War), and 2001 (9/11, Afghanistan), it is still patriotic in 2011.