It's hard to comprehend why Senator Bob Corker, who famously said Donald Trump did not have the stability to be a successful president last year and then suggested the White House had become "an adult day care centre" felt the need to go public with the fact he had been offered the job, and turned it down, this week.

One would have thought, given Australia has not had an American ambassador appointed since the departure of Obama appointee, John Berry, in 2016, discretion would have been the better path to follow.

Mr Corker's decision not to accept what would once have been considered one of America's plum diplomatic postings appears to be tied up with his own domestic political situation.

While the 65-year-old has previously announced his decision to retire from the U.S. Senate at the end of his second six-year term in December, this no longer appears to be set in stone; no matter how much President Trump, who offered him the Canberra post four weeks ago, would like him to.

"Right now we're kind of sprinting towards the finish line (of his work in the Senate)," Senator Corker told Reuters on Monday.

"There may be some other task that needs to be done down the road, and certainly I have always honoured public service and we'll see what happens."

Asked to explain why he knocked back the Australian ambassadorship the Senator, who has apparently never visited this country, said would not be comfortable in the role.

"At the end of the day I just felt like it wasn't the right fit," he said.

His very public refusal highlights the lack of diligence the Trump administration has brought to filling key ambassadorial positions around the world.

As of April 38 U.S. ambassadorial positions were vacant. They included Turkey, Sweden, Saudi Arabia, Egypt, Libya, South Africa, Ireland, Belgium and Mongolia.

The post of U.S. ambassador to the European Union, which has more than 500 million citizens - at least until Brexit takes effect - and is America's largest trading partner, is also available.

While Australia had been on track to have an American ambassador by Christmas with news of the appointment of Admiral Harry Harris, the head of the US Pacific Command, in January, all that changed last month.

The admiral was reassigned to South Korea following the recent remarkable developments on the peninsula. While that appointment was generally welcomed it was queried by a number of observers, including the Centre for Strategic and International Studies' Andrew Shearer.

It was, he said, "hard to escape a bit of a sense that Australia is being treated here as a second class ally".

That is a view more and more Australians are likely to share unless Mr Trump is able to find a willing candidate to fill the vacancy in the very near future.

The only good news to emerge from the Corker fiasco is that the Donald Trump administration is apparently now hard at work on the case.