Jim Murphy, Donald Trump’s national political director, is taking a step back from the campaign for personal reasons.

"I have not resigned but for personal reasons have had to take a step back from the campaign," Mr Murphy told Politico on Thursday without elaborating.

The development could mean trouble for the campaign as the election is just 19 days away and Mr Trump’s popularity has taken a hit in recent weeks.

Mr Murphy has played a key role in establishing field programs in battleground states. According to Politico, there’s widespread concern amongst Republicans that the party’s nominee is far behind the Clinton campaign when it comes to swing state infrastructure.

Mr Murphy is a close ally of Paul Manafort, the former chairman of the Trump campaign. Mr Manafort resigned in August amid growing concerns over his ties to Ukraine politics.