Vice president Mike Pence has generally been a consistent source of stability in a Trump administration renown for chaos, but he swapped roles with the president this week after he abruptly cancelled a planned trip to New Hampshire.

Mr Pence had reportedly already boarded Air Force Two at Joint Andrews Base in Maryland when he abandoned his trip to Salem, where he was due to participate in a roundtable discussion and deliver remarks on the opioid crisis.

But Mr Pence never arrived in New Hampshire, and rumours swirled in Washington about the possible cause of the cancellation. It is rare that something causes a vice president to abandon plans that have previously been publicised.

The reason for the cancellation was not immediately clear, and initially, the announcer at the event in Salem told the crowd that Mr Pence would no longer participate in the roundtable because of an "emergency callback."

“Something came up that required the @VP to remain in Washington, DC. It’s no cause for alarm. He looks forward to rescheduling the trip to New Hampshire very soon,” wrote Mr. Pence’s press secretary, Alyssa Farah, on Twitter.

Vice President Mike Pence was due to deliver remarks about the opioid crisis (Alamy)

Theories that speculated on the reason for the cancellation, from fears of a national security risk to personal drama involving Donald Trump, were abound on Twitter for about an hour until Ms Farah provided an update.

“The @VP never left Washington, DC. There was no ‘emergency callback.’ Something came up that required the VP to stay in DC. We’ll reschedule NH shortly,” Ms Farah tweeted. She did not offer further clarifications.

The comments made by the Salem announcer were later denied by Mr Pence's aides, who said there was no national security emergency or heath scare that prompted the change, and that though he boarded the plane it never took off.

The aides offered little else in the way of specific detail, and Mr Pence's chief of staff, Marc Short, would only rule out possibilities to reporters at the White House without detailing what may have actually happened.

“When would people know what happened?” reporters asked. “Weeks from now,” Mr Short replied, according to a pool report.