Republican U.S. Sen. Susan Collins says new revelations in a book draft by former national security advisor John Bolton strengthen the case for witnesses in President Trump's ongoing impeachment trial.

In a statement Monday, Collins reiterated her position that witnesses in the trial should be called after House managers and President Donald Trump's attorneys have made their cases.

My statement on Bolton developments. pic.twitter.com/3M59J7suts — Sen. Susan Collins (@SenatorCollins) January 27, 2020

But Collins and several other potential Republican swing voters have been under pressure to push for witnesses now.

That pressure is growing after a New York Times report that says Bolton's upcoming book details how the president wanted to continue freezing security assistance to Ukraine until the country agreed to investigations into Trump's political rivals.

The revelations could torpedo a key defense made by the president's attorneys - that the aid delay was not related to his pursuit of investigations.

Maine independent U.S. Sen. Angus King could not immediately be reached for comment, but he's repeatedly argued that Bolton and other witnesses should be called now.

This story will be updated.