It takes a lot for C-Span to get mad.

But an unusual move by the South Carolina Democratic Party to bar the network from putting on a live broadcast of its annual convention — which is doubling as a major showcase for 21 presidential candidates — has left executives at the strait-laced public affairs network fuming. CNN and Fox News aren’t thrilled about it, either.

The source of the friction is a deal struck between the state party and MSNBC, the liberal-leaning cable network, granting the channel exclusive rights to show the candidates’ speeches live on Saturday.

Under the party’s rules, which were abruptly announced this week, rival television networks will have to wait three hours after the event concludes before broadcasting their footage.

News organizations always scrap for exclusive interviews with prominent politicians. But the South Carolina convention, virtually a required stop for presidential hopefuls, is typically open to all journalistic comers. The decision to restrict coverage set off broader concerns that the state party was picking and choosing the news organization allowed to cover what is a crucial event in an early-voting state.