That would be Mark Pryor, first-term Democratic incumbent, running in a reliably red state.

[T]he Republicans’ inability to field any Senate candidate in a Southern state that twice favored Republican George W. Bush for president this decade is yet another blow for a party that lost six seats and its Senate majority in 2006, and is mainly playing defense against further Democratic gains this year.

Having missed the filing deadline, any Republican who might belatedly decide to run against Pryor would have to do so as a write-in candidate…

Pryor becomes the first senator to draw no opponent from the other major party since 2006, when Indiana Republican Richard G. Lugar enjoyed a free ride in his campaign for a sixth term.

Be sure to take a moment to send an e-mail to the NRSC to thank them for the bang-up job they did here in making the Democrats work for their majority. Exit question: You-know-who got the last laugh after all, didn’t he? Heart-ache.

Update (Ed): Beyond the disappointment of the story’s subject matter, I have to marvel at the writing.  Pryor’s the first Senator to run unopposed … since 2006?  How many elections is that?  Answer: none.  When Lugar ran unopposed in 2006, did people write that he was the first since 2004?