"In 1997 a number of House members wanted to throw him out as Speaker," Dole wrote. "But he hung on until after the 1998 elections when the writing was on the wall. His mounting ethics problems caused him to resign in early 1999. I know whereof I speak as I helped establish a line of credit of $150,000 to help Newt pay off the fine for his ethics violations. In the end, he paid the fine with money from other sources."
Gingrich has characterized the penalty from the ethics panel as reimbursement for their investigation, which centered on a college course he taught that was misclassified as tax-exempt.
Dole drew on his own experience with the former House speaker to paint the candidate as lethal for the Republican Party.
"If Gingrich is the nominee it will have an adverse impact on Republican candidates running for county, state, and federal offices," Dole said. "Hardly anyone who served with Newt in Congress has endorsed him and that fact speaks for itself. He was a one-man-band who rarely took advice. It was his way or the highway."
Dole, who served in the U.S. Senate when Gingrich was House speaker, harkened back to Gingrich's style during his time in Congress.
"Gingrich had a new idea every minute and most of them were off the wall," Dole said. "He loved picking a fight with Bill Clinton because he knew this would get the attention of the press. This and a myriad of other specifics helped to topple Gingrich in 1998."
Dole endorsed Romney in December, and made calls to allies in Iowa urging them to support Romney ahead of the state's first-in-the-nation caucuses. Dole won Iowa in 1996, but ultimately lost the general election to Bill Clinton.
In his letter, Dole placed a measure of blame on Gingrich for the loss.
"In my run for the presidency in 1996 the Democrats greeted me with a number of negative TV ads and in every one of them Newt was in the ad. He was very unpopular and I am not only certain that this did not help me, but that it also cost House seats that year."
Dole pointed to a particular eccentricity of Gingrich's, saying the former speaker's style was as puzzling as it was harmful.
"Newt would show up at the campaign headquarters with an empty ice-bucket in his hand-that was a symbol of some sort for him-and I never did know what he was doing or why he was doing it," Dole wrote.
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