"I think it could've been with the debate," Trump admitted in New Hampshire when asked about his second-place finish.
"I think some people were disappointed that I didn't go into the debate," he was quoted as saying by Fox News.
Despite leading in most of the pre-caucus polls, the 69-year-old real estate billionaire finished second behind Texas Senator Ted Cruz and barely held off Florida Senator Marco Rubio, who came a close third, opening up a three-way contest in the Republican camp.
Trump announced a rival event to benefit Veteran Organisations in Des Moines exactly at the same time as the debate.
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He shunned the last primary debate before the Iowa caucuses but relished the drama triggered by his snub of the top US news network and taunted his rivals at the competing fundraising event schemed to chide them.
The fundraiser for veterans raised USD 6 million. Trump asserted that he would make the same decision again, saying he "would never, ever give that up (fundraiser for veterans) to go between first and second in Iowa."
Trump was also quoted as saying that his campaign "didn't have much of a ground game because I didn't think I was going to be winning."
"In retrospect, we could have done much better with the ground game," Trump said, in an apparent slight to his Iowa state director Chuck Laudner.
Laudner had said in January he felt "fantastic about the ground game."
Trump is known for making controversial comments and has made quite a few assertions during his campaign that have attracted criticism. He had called for a ban on Muslims entering the United States, triggering sharp reactions from across the world.