The endorsement could give Sanford's flagging campaign a boost heading into its final stretch — especially with fundraising, a key problem for the former governor against a well-funded opponent.
While South Carolina's 1st district leans Republican Sanford has struggled with self-inflicted wounds. While Colbert Busch is getting big-money support from national Democrats, the National Republican Congressional Committee and GOP outside groups have refused to help him.
Sanford and Paul have similar political pedigrees, as hardline fiscal conservatives who often buck their own party, and served together in the House in the 1990s.
The two also share a key ally: Sanford's close friend, South Carolina state Sen. Tom Davis (R), is active in the Liberty Movement and headed Paul's South Carolina presidential campaign.
Paul also endorsed Rep. Paul Broun's (R-Ga.) Senate run on Thursday.
This post was updated at 5:40 p.m.