Billionaire US Republican presidential candidate Donald Trump says he plans to spend at least $2m a week on television advertising in the first three voting states, a move that would mark a departure for a candidate who has so far relied on free media to fuel his insurgent campaign.

Trump told reporters on his private jet on Tuesday that he did not want to take anything for granted.

“I don’t think I need to spend anything. And I’m very proud of the fact that I’ve spent the least and achieved the best result,” Trump told reporters before a rally in Council Bluffs, Iowa. “I feel I should spend. And honestly I don’t want to take any chances.”

Trump, who leads in national Republican preference polls, has seen his lead dissolve in Iowa, where Texas senator Ted Cruz is surging on the backing of the state’s robust evangelical conservative voting bloc.

The frontrunner has spent nothing on television advertising to date, and just over $300,000 on radio ad time, according to advertising tracker Kantar Media’s CMAG – far less than his rivals. Former Florida governor Jeb Bush, for example, has spent more than $40m, but trails Trump and several other candidates.

Trump has considered advertising on television in the past, saying he is willing to spend whatever it takes to win the Republican nomination, but so far he has proven a frugal campaigner, putting little of his own money on the line.

While Trump likes to claim he’s self-funding his campaign, the vast majority of what he has spent has come from donors across the country sending cheques or purchasing merchandise from his website.

“I’ll be spending a minimum of $2m a week and perhaps substantially more than that,” he told reporters, adding, “If somebody attacks me, I will attack them very much and very hard in terms of ads.”

Trump said he had screened the first two ads, saying they touch on immigration, trade and national security policy.

Since the real estate mogul announced his candidacy in June, his tendency for combative language has resulted in a steady stream of what he himself has termed “free nationwide publicity”, often stemming from comments made during the five Republican debates or during his speeches at packed rallies.

Trump says he believes the thousands who attend his rallies, like the more than 3,000 in Council Bluffs, will turn out to vote for him. Iowa’s caucuses begin the 2016 voting on 1 Febuary.