From http://en.wikipedia.org/wiki/Piss_Christ :
Originally Posted by Soeren
"The piece caused a scandal when it was exhibited in 1989, with detractors accusing Serrano of blasphemy and others raising this as a major issue of artistic freedom. On the floor of the United States Senate, Senators Al D'Amato and Jesse Helms expressed outrage that the piece was supported by the National Endowment for the Arts, since it is a federal taxpayer-financed institution."
In that specific case, Christian conservatives specifically and loudly expressed their opinion that the NEA should be only supporting art that supported their interpretation of religious beliefs (notably, at least one Catholic nun supported Serrano, because her interpretation was different). The result was to threaten the work and livelihoods of all artists whose work was publicly supported.