United States senator Bob Corker has eviscerated President Donald Trump repeatedly on national television, calling him a liar who has "debased the country" in an extraordinary diatribe against a sitting president by a fellow Republican.

Key points: Corker says "debasement" of America is what Trump will be remembered for

Corker says "debasement" of America is what Trump will be remembered for He says world leaders are aware the US President blatantly lies

He says world leaders are aware the US President blatantly lies Trump hits back that Corker made the US a global joke with the Iran deal

Trump hits back that Corker made the US a global joke with the Iran deal Another senator says he won't run next year as politics has become "undignified"

Senator Corker, who is not seeking re-election next year, accused the President of telling falsehoods that could easily be proven wrong and of wilfully damaging the country's standing in the world, exposing deepening divisions in the Republican Party under Mr Trump's White House.

"The President has great difficulty with the truth on many issues," Senator Corker said in a CNN interview at the Capitol, before Mr Trump was due to meet with senators to seek consensus on proposed tax cuts.

"It's amazing. Unfortunately world leaders are very aware that much of what he says is untrue," Senator Corker continued, in the sharpest of his bitter public exchanges with the President in recent weeks.

"Certainly people here are because these things are provably untrue. They're just factually incorrect and people know the difference."

Shortly after, Mr Trump was also attacked in a dramatic speech on the Senate floor by Republican Jeff Flake, who said American politics had become inured to "reckless, outrageous and undignified" behaviour from the White House.

"The instinct to scapegoat and belittle threatens to turn us into a fearful, backward-looking people," said Senator Flake, who announced he would not run for re-election next year.

Sorry, this video has expired Republican senator lashes out at leadership

"I will not be complicit or silent."

Senator Flake criticised the "flagrant disregard of truth and decency" that he said is undermining American democracy.

Speaking later to reporters, Senator Corker called Mr Trump a bully.

Mr Trump hit back on Twitter after Senator's Corker's television interviews, calling the Senate Foreign Relations Committee chairman "incompetent" and reprising one of his trademark derisive nicknames.

Loading

"He doesn't have a clue as the entire World WAS laughing and taking advantage of us. People like liddle' Bob Corker have set the US way back," Mr Trump tweeted.

Although he is a foreign policy specialist, Senator Corker is also a key player in the tax debate.

His support could be crucial as Republicans seek passage of the White House-backed tax plan in the closely divided Senate.

Loading

The senator from Tennessee, whose recent announcement he would not seek re-election in November 2018 has freed him from the need to stick to a voter-friendly script, pulled no punches in his onslaught against Mr Trump.

He acknowledged that tensions between the two men, once allies, have been building for months.

While Senator Corker's comments were the strongest yet from a fellow Republican, Mr Trump has also provoked the ire of another respected senior Republican, John McCain, whose war record he mocked.

Mr McCain's barbs prompted Mr Trump last week to tell the 80-year-old senator, who has brain cancer, to be careful because, "I fight back".

Trump 'purposely breaking down' global ties

The latest exchange began overnight when Senator Corker advised the White House not to interfere in lawmakers' tax deliberations and called Mr Trump's lunch a "photo op".

Later, Mr Trump took to Twitter to belittle Senator Corker, a national security adviser on his presidential campaign, with an early morning tweet in which he anticipated opposition for his administration's tax plan.

Loading

That prompted a retort on Twitter from Senator Corker.

Loading

In his CNN interview, Senator Corker assailed Mr Trump for telling falsehoods that are easily disproven and questioned why he persisted in doing so.

"I don't know why he lowers himself to such a low, low standard and debases our country in that way but he does," he said.

"He is purposely breaking down relationships we have around the world that had been useful to our nation.

"I think the debasement of our nation is what he'll be remembered most for."

With the tax plan, Mr Trump is hoping to notch up his first major legislative win — something that has so far eluded him, even though Republicans control the White House and both chambers of Congress.

Bob Corker campaigned with Donald Trump during last year's election. ( Reuters: Joshua Roberts )

Reuters