GOP presidential candidate Herman Cain says he is 'not deterred' by political attacks and is aiming to win the Iowa caucuses. 

In a fundraising email sent out to supporters on Thursday, Cain acknowledged it is "getting ugly out there" and that his campaign needs money to defend the candidate from opponents, whom the Republican has accused of leaking to the media allegations he sexually harassed women at a trade group in the 1990s. 

"The American political establishment has never seen a campaign like ours. It terrifies them," Cain said in the letter. "So my opponents are attacking me."

Cain on Wednesday accused rival Rick Perry of planting the report about the alleged sexual harassment, and Cain Chief of Staff Mark Block said the Perry campaign should apologize to Cain. Perry's campaign has denied even knowing of the allegations before they became news.

"Already they are running negative ads against me on TV," Cain said of his opponents. "They are attacking me through the media. It's getting ugly out there. But I'm not deterred."

Cain said the attacks against him are from Republicans, Democrats and members of the press, and that he must "win Iowa!" 

"My opponents already have millions of dollars in the bank that they plan to use to blanket Iowa with negative ads," Cain continued in the email. "My campaign must be prepared to defend itself and to get our positive message out."

Cain is under fire for reports the National Restaurant Association agreed to financial settlements with two women who accused Cain of sexually inappropriate behavior when he was president of the trade group. Cain acknowledged one settlement, but said he was falsely accused. 

The former CEO of Godfather's Pizza is a surprise contender for the Republican presidential nomination and has surpassed Perry and former Massachusetts Gov. Mitt Romney in some polls. 

Romney's campaign also denied having anything to do with the story. 

"We had absolutely nothing to do with it," Romney spokeswoman Andrea Saul said Monday to The Hill.