President Trump strenuously denied a claim by national security adviser John Bolton that he told Bolton he was withholding aid from Ukraine for political reasons.

In leaked portions of his upcoming book, The Room Where it Happened, Bolton, 71, claimed that Trump told him he would not release military aid to Ukraine until officials there announced an investigation into Joe Biden and Hunter Biden. The details from the leaked memoir prompted several top Democrats to demand that Bolton testify before the conclusion of the Senate’s impeachment trial.

In a series of tweets early Monday morning, Trump claimed his former adviser was only making the allegations because they could boost book sales.

“I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens,” Trump tweeted. “In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book.”

He continued, “With that being said, the transcripts of my calls with President Zelensky are all the proof that is needed, in addition to the fact that President Zelensky & the Foreign Minister of Ukraine said there was no pressure and no problems.”

Trump added that he believes he did more to protect Ukraine than Barack Obama, “Additionally, I met with President Zelensky at the United Nations (Democrats said I never met) and released the military aid to Ukraine without any conditions or investigations — and far ahead of schedule. I also allowed Ukraine to purchase Javelin anti-tank missiles. My Administration has done far more than the previous Administration.”

I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book. With that being said, the... — Donald J. Trump (@realDonaldTrump) January 27, 2020

...(Democrats said I never met) and released the military aid to Ukraine without any conditions or investigations - and far ahead of schedule. I also allowed Ukraine to purchase Javelin anti-tank missiles. My Administration has done far more than the previous Administration. — Donald J. Trump (@realDonaldTrump) January 27, 2020

In a separate tweet, Trump addressed the calls from Democrats for Bolton to testify, writing, “The Democrat controlled House never even asked John Bolton to testify. It is up to them, not up to the Senate!”

The Democrat controlled House never even asked John Bolton to testify. It is up to them, not up to the Senate! — Donald J. Trump (@realDonaldTrump) January 27, 2020

The Senate debate over calling additional witnesses has been brewing since the rushed impeachment process in the House concluded. House Speaker Nancy Pelosi fast-tracked impeachment and refused to wait for court orders that could have allowed the House to hear from witnesses such as Bolton. The former adviser said he would be willing to testify before the Senate, but Republican leaders, including Senate Majority Leader Mitch McConnell of Kentucky, have shown little interest .

However, Maine Republican Sen. Susan Collins said she and a “small group” of Republicans who are open to calling additional witnesses. It is not clear that she has enough support among Republicans to cross the 50-vote threshold need to win a vote on calling witnesses.