US senators have postponed a vote on whether to approve Donald Trump's nominee to lead the Immigration and Customs Enforcement (ICE) agency after a union raised concerns about his “judgement and professionalism”.

The Senate Homeland Security Committee chairman Ron Johnson said there were “some issues” with the proposed permanent appointment of acting director Ronald Vitiello, but declined to elaborate.

It is the second time the committee had delayed a vote on the nomination. It was initially scheduled for November last year but was postponed after the National ICE Council, which represents agency staff, first raised concerns about Mr Vitiello’s fitness to lead the agency.

On Tuesday, union president Chris Crane sent a letter formally asking the panel to oppose the nomination. He cited tweets which he said showed Mr Vitiello “lacks the judgment and professionalism to effectively lead a federal agency", including one post which compared Democrats to the Ku Klux Klan.

In the 2015 tweet, Mr Vitiello suggested the Democratic Party should be renamed “NeoKlanist,” a reference white supremacist group. In another social media post, he compared Mr Trump – then a presidential candidate – to the troublemaking Dennis the Menace comic character.

Mr Crane suggested such comments violated official codes of conduct at the Department of Homeland Security and could jeopardise criminal cases because they could be used by the defence in a trial to impeach ICE’s credibility.

“This type of conduct would result in a rank and file ICE employee being disciplined, if not possibly removed from employment,” the union president added.

In his confirmation hearing on 15 November, Mr Vitiello told senators he had been “trying to make a joke” and regretted the tweets, which he said intended to send as direct messages.

Vincent Picard, an ICE spokesman, said the posts were made from a private account while Mr Vitiello was off-duty, and that the Office of Special Counsel (OSC) had cleared him of any wrongdoing.

“The National ICE Council’s focus on a couple of old tweets that have been investigated and addressed is a disservice to the men and women of ICE,” he added.

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Mr Vitiello is already likely to face opposition from Democrats after refusing to rule out reinstating Mr Trump’s child separation policy.

Announcing the postponement of the vote on Wednesday, Mr Johnson said: “We are going to hold over the ICE director nomination.