The president tweeted: “The call to the Ukrainian President was PERFECT. Read the Transcript! There was NOTHING said that was in any way wrong. Republicans, don’t be led into the fools trap of saying it was not perfect, but is not impeachable. No, it is much stronger than that. NOTHING WAS DONE WRONG!”
The message appeared to be aimed at supporters including the Kentucky senator Rand Paul who have been putting forward an alternative defence to his own.
Mr Trump has repeatedly claimed that his 25 July call to the Ukrainian president Volodymyr Zelensky, in which he asked him to investigate his political rivals, was “perfect”.
He has been insistent that the call did not contain a “quid pro quo”, namely that he was withholding military aid and a White House visit until the Ukrainians announced an investigation into Democratic presidential hopeful Joe Biden.
The call prompted an anonymous whistleblower complaint which in turn led to an impeachment inquiry.
Mr Trump has said the aid was being withheld because he was concerned at corruption in Ukraine and because he thought the European Union should be contributing more to defend the former Soviet nation against Russian-backed separatists.
Appearing on NBC’s Meet the Press on Sunday, Mr Paul offered an alternative defence, suggesting the president should stop denying there had been a quid pro quo.
He said: “Presidents have withheld aid before for corruption.
“So the thing is, I think it’s a mistake to say, ‘Oh, he withheld aid until he got what he wanted’. Well, if it’s corruption and he believes there to be corruption then he has every right to withhold aid.
“So I think it’s a big mistake for anybody to argue ... ‘he didn’t have a quid pro quo’, and I know that’s what the administration’s arguing. I wouldn’t make that argument. I would make the argument that every politician in Washington, other than me virtually, is trying to manipulate Ukraine to their purposes.”
Another Republican, congressman Mac Thornberry, told ABC’s This Week that while it was inappropriate for Mr Trump to have asked a foreign country to help discredit a political opponent, he should not be impeached for it because it’s the way he acts “all the time”.
He said: “There’s not really anything the president said in that phone call that’s different to what he says in public all the time.
“So is there some sort of abuse of power that rises to that threshold that is different than the American people have been hearing for three years? I don’t hear that.”
On Friday, the former US ambassador to Ukraine, Marie Yovanovitch, will give evidence.
While Mr Trump has consistently claimed that the partial transcript of the 25 July call released by the White House shows the call was “perfect” -- repeatedly tweeting the phrase “read the transcript” -- Democrats believe it shows him linking military aid to a “favour”, namely investigations into his political rivals.
Mr Trump has repeatedly called for the whistleblower, who enjoys legal protection from being identified, to be unmasked and cross-examined.
Mr Graham said: “It’s impossible to bring this case forward in my view fairly without us knowing who the whistleblower is and having a chance to cross examine them about any biases they may have. So if they don’t call the whistleblower in the House, this thing is dead on arrival in the Senate.”
The Democrat-controlled House of Representatives is expected to approve articles of impeachment -- essentially charges of wrongdoing -- against the president. The Senate, which is controlled by Republicans, would then be expected to hold an impeachment trial to decide whether Mr Trump should be removed from office.