WASHINGTON (AFP) - Less than a month ago, US Secretary of State Mike Pompeo triumphed over rivals to emerge as the dominant force in United States President Donald Trump's foreign policy.

Mr Pompeo's ascendancy was confirmed in a very public manner when Mr Trump sacked his chief adversary, Mr John Bolton, as national security adviser on Sept 10.

But Mr Pompeo's fortunes have suddenly darkened over his role in the Ukraine scandal that has triggered an impeachment probe into the President.

After prevaricating for days, Mr Pompeo finally confirmed he was on the now-famous telephone call when Mr Trump pressed Ukraine for damaging information on Mr Joe Biden - whom Mr Trump may face in the 2020 election.

Mr Pompeo had previously dodged questions about the call in an interview with ABC News on Sept 22, suggesting he knew nothing about the conversation.

Last week, he said he had read only "the first couple of paragraphs" of a whistle-blower's complaint about the July 25 call - before curtly confirming press reports on Wednesday (Oct 2) that he had been on the call.

Mr Pompeo's Democratic critics say he is now a "witness" caught in a conflict of interest that should rule him out of decisions on how the State Department deals with the investigation.

He has been accused of "stonewalling" the investigation and trying to limit what his staff could discuss if they testify.

'DESTRUCTION OF US DIPLOMACY'

Mr Pompeo himself has been subpoenaed by the Congressional committees carrying out the impeachment investigation for documents relating to the Ukraine scandal, putting him in the firing line of the political row that threatens to engulf Mr Trump.

Mr Trump's personal lawyer Rudy Giuliani has said he gave a dossier of Ukraine documents directly to Mr Pompeo in March.

According to the House committees investigating possible impeachment, the dossier was "a package of disinformation, debunked conspiracy theories, and baseless allegations".

The Washington Post this week accused Mr Pompeo of "enabling the destruction of US diplomacy".

"Mr Pompeo did nothing to stop Mr Giuliani from allying himself with some of Ukraine's most corrupt figures to peddle false stories about Mr Biden, as well as conspiracy theories about Ukraine's role in the 2016 presidential election," it said in an editorial.

One potential threat to Mr Pompeo - and his boss - could be the evidence of former US special envoy to Ukraine Kurt Volker, who resigned last week.

The whistle-blower complaint depicts Mr Volker as speaking with Mr Giuliani to "contain the damage" of the lawyer's meddling in Ukraine affairs.

A day after the July 25 call, Mr Volker met Ukraine President Volodymyr Zelensky and other senior Ukraine officials and advised them how to "navigate" Mr Trump's demands.

Mr Pompeo, a former soldier, lawyer and businessman, has so far taken a characteristically defiant stance - sticking loyally by the President and dismissing his critics with contempt.

When the House committee chairs asked for five diplomats to testify, Mr Pompeo blasted the request as "an attempt to intimidate, bully, and treat improperly the distinguished professionals of the Department of State".

Setting the scene for a campaign to prevent or delay any impeachment proceedings, Mr Pompeo has accused the lawmakers of "serious substantive and procedural deficiencies" in their request for testimonies.

TRUMP'S CLOSE ALLY

Mr Pompeo, so close to Mr Trump that the President last year said he was the only adviser with whom he has never argued, has risen quickly in Washington since arriving as a Kansas congressman elected in the 2010 right-wing populist "Tea Party" movement.

Many speculate he could be enticed by an opening to represent Kansas in the Senate next year - perhaps with an eye on running for the top prize in the 2024 presidential election.

Ironically, the impeachment fight echoes how Mr Pompeo came to prominence pursuing former secretary of state Hillary Clinton and the State Department in the inquiry into the deadly 2012 attack on the US consulate in Benghazi.

At the time, he said "the White House in particular left large holes in the investigation by denying the Committee access to documents and witnesses". His critics now hope to hold his words against him.