The knives are out for the improbable early star of the 2020 Democrat presidential contest.

Pete Buttigieg, the 37-year-old mayor from Indiana, has stunned his fellow candidates by surging into third place in some polls, having been unheard of only weeks ago.

His meteoric rise has left flat-footed the shadowy political dirt-diggers who operate on the fringes of US political campaigns.

They had gathered plenty of damaging information - officially known as "opposition research" - on big name rivals, but knew nothing of Mr Buttigieg.

That is about to change as researchers head for South Bend, his home town, where they plan to look through every budget Mr Buttigieg ever passed, read the minutes of every council meeting he was ever at, and file Freedom of Information requests for anything not public.

Already, a theme is emerging which will be used to attack the young mayor - that black voters will not support him.

Black voters make up a key constituency in the Democrat nomination process.

In South Carolina, the first state in the South to go to the polls, around 60 per cent of Democrat primary voters are black.

Mr Buttigieg's crowds so far have been overwhelmingly white and noticeably lacking in racial diversity.