Washington (CNN) Senior White House officials were certain Vice President Mike Pence would back an effort to have President Donald Trump removed from office over concerns about his mental state, the "Anonymous" administration official plans to claim in their forthcoming book, a source close to the book process confirms to CNN's Jake Tapper.

The book doesn't say Pence actually said he would support such a move, according to a summary of an excerpt first reported by HuffPost , which the source described as accurate.

The Vice President dismissed the report Thursday and called on the book's author -- who has been described as a senior Trump administration official who previously alleged there's an internal administration resistance to Trump in a 2018 New York Times op-ed -- to resign.

"When those rumors came out a few years ago, I dismissed them then. I never heard any discussion in my entire tenure as vice president about the 25th Amendment , and why would I?" Pence told reporters in New Hampshire.

He called the anonymous administration official "appalling."

"I have to tell you, this the very notion of this anonymous who wrote an editorial, and now it's reported that they've written a book, is just appalling to me. If there's someone in our administration or served in our administration who doesn't support this President, doesn't support his agenda, they should do the honorable thing and resign," he said.

The book, "A Warning," will be released November 19, and the author's intent is to convince the nation to not reelect Trump in 2020, CNN previously reported . The author first publicly came to light when they wrote the op-ed in September 2018 that alleged a "resistance" inside the Trump administration and described a President the author viewed as amoral, opposed to many conservative values such as freedom, and "impetuous, adversarial, petty and ineffective." The author wrote that "many of the senior officials in his own administration are working diligently from within to frustrate parts of his agenda and his worst inclinations."

Sources familiar with the book have previously told CNN the publisher and the author's literary agents at Javelin were provided verification that the author of the book and op-ed is the same person.

In the op-ed, the author said "there were early whispers within the cabinet of invoking the 25th Amendment." In February, former acting FBI Director Andrew McCabe said former Deputy Attorney General Rod Rosenstein raised high-level discussions at the Justice Department about recruiting Cabinet members to invoke the 25th Amendment in the aftermath of former FBI Director James Comey's firing in spring 2017. Rosenstein, through a Justice Department spokesperson, has repeatedly disputed McCabe's characterization of his remarks, though he has not denied the topic was discussed at some point.

The book's author says they were a frequent participant in meetings with the President and had access to internal notes they plan to include in their book.

"In these pages, you will not just hear from me. You will hear a great deal from Donald Trump directly, for there is no better witness to his character than his own words and no better evidence of the danger he poses than his own conduct," the book's back cover reads.