WASHINGTON (Reuters) - U.S. Senator Bob Corker eviscerated President Donald Trump repeatedly on national television on Tuesday, calling him a liar who has debased the country in an extraordinary diatribe against a sitting president by a fellow Republican.

Corker, the only Senate Republican not seeking re-election next year, accused the president of telling falsehoods that could easily be proven wrong and of willfully damaging the country’s standing in the world, exposing deepening divisions in the Republican Party under Trump’s White House.

“The president has great difficulty with the truth on many issues,” Corker told CNN in the first of a series of interviews at the Capitol hours before Trump was due to meet with senators to seek consensus on proposed tax cuts.

“Unfortunately world leaders are very aware that much of what he says is untrue,” said Corker, 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.”

Speaking later to reporters, Corker called Trump a bully.

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

“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 U.S. way back,” Trump tweeted.

Although he is a foreign policy specialist, 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. [nL2N1MZ0SH]

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

The Trump-Corker clash is the latest to erupt in what has been a sometimes fraught relationship between the president and the politicians who should be his partners in Congress.

Trump pilloried Senate Republicans - as a group and by name - after their failure this summer to repeal and replace the 2010 healthcare law known as Obamacare, one of his top presidential campaign promises.

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

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

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The latest exchange began early Tuesday morning when Corker advised the White House not to interfere in lawmakers’ tax deliberations and called Trump’s lunch a “photo op.”

Later, Trump took to Twitter to belittle 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.

“Bob Corker, who ... couldn’t get elected dog catcher in Tennessee, is now fighting Tax Cuts,” Trump wrote.

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That prompted a retort on Twitter from Corker, “Same untruths from an utterly untruthful president.”

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

TWEETS CAUSE TENSIONS

Tensions with Corker erupted this month when Trump said Corker “begged” for his endorsement as he weighed running for re-election and Corker responded by calling Trump’s White House an “adult day care center.”

Trump has also blamed Corker for the Iran nuclear deal forged under Democratic President Barack Obama. Every Republican, including Corker, voted against the 2015 accord.

In his CNN interview, Corker assailed 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 but he does.”

“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.”

Corker, who was an early backer of Trump’s White House campaign at a time when many senior Republicans were expressing doubts, said he would not support him again.

“I was one of those - that hoped he would rise to the occasion as president and aspire to lead our nation instead of divide it,” he told reporters at the Capitol. “He has not risen to the occasion.”