An all-but-unknown Democrat has soundly beaten his Republican rival to become the next governor of Louisiana, in a stunning political upset that went from impossible to inevitable over the course of the campaign.

In a run-off election this weekend, State Representative John Bel Edwards trounced US Senator David Vitter – once considered a shoo-in for the job in an otherwise deep-red state – with 56 per cent of the vote.

Mr Edwards will take over from the Republican incumbent, Bobby Jindal, in January, becoming the only Democratic governor in the Deep South. A staunch Catholic who served in the US army, he will be out of step with much of his party on issues including gun rights, which he supports, and abortion, which he opposes. But while his conservative credentials undoubtedly won him votes, Mr Edwards’s victory was also assured by the implosion of his opponent’s campaign.

Mr Vitter, a US Senator since 2005, was tainted by scandal in 2007 when his phone number was found on a list kept by Deborah Palfrey, the so-called “DC Madam”, who was convicted of running a prostitution ring in the US capital. Mr Vitter, who apologised for his “very serious sin” at the time, won re-election to the Senate in 2010, but the scandal returned to haunt his gubernatorial run.

Last month, an already bitter race became further charged after the journalist Jason Berry published an interview with Wendy Ellis, a former escort who claimed she and Mr Vitter had a lengthy affair and that she gave birth to his child, despite Mr Vitter urging her to get an abortion. Mr Vitter’s campaign was forced to deny Ms Ellis’s claims, saying they had “zero legitimacy”.

Mr Edwards and Mr Vitter had advanced to the run-off following the state’s open “jungle primary” last month, in which candidates of all parties run against each other, with the top two contesting a run-off if neither secures 50 per cent of the overall vote.

Already damaged by the various vice allegations, Mr Vitter ended the primary 14 points behind Mr Edwards. In conceding his final defeat on Saturday, Mr Vitter also announced that he would not run for re-election to the Senate next year. Noting the nastiness of the campaign in his victory speech, Mr Edwards said voters in Louisiana had “chosen hope over scorn, over negativity and over the distrust of others”.

Lousiana’s last Democratic governor, Kathleen Blanco, was succeeded by Mr Jindal in 2008. Mr Jindal, prevented by term limits from running again, has grown increasingly unpopular over his tenure, due in large part to Louisiana’s budget woes: the state had a surplus of almost $1bn when Mr Jindal took office; this year there was a $1.6bn shortfall.

Mr Jindal also struggled to muster even 1 per cent support in national polls during his run for the 2016 Republican presidential nomination, which he terminated last week.