Paul Manafort, Trump's campaign chairman, also insisted Sunday on ABC's "This Week" that his campaign is "not worried about fundraising" and won't be outspent by Hillary Clinton.
And he said Trump won't apologize for appearing to mock a disabled reporter in November 2015 -- footage now appearing in a pro-Clinton super PAC's anti-Trump attack ad -- saying that Trump has "already dealt with that matter. He dismissed it."
"He wasn't mocking him. And he's said that in the past. He said he was not making reference to him. And so he's dealt with that issue," Manafort said.
Trump also pushed back on the ad, tweeting Sunday morning: "Clinton made a false ad about me where I was imitating a reporter GROVELING after he changed his story. I would NEVER mock disabled. Shame!"
His comments come as Republicans tread carefully around their party's 2016 standard-bearer.
Trump's criticism of Judge Gonzalo Curiel -- asserting that the Indiana-born judge's Mexican heritage, combined with Trump's push to build a U.S.-Mexico border wall, has left him biased in the Trump University lawsuit -- led to harsh rebukes from many Republicans.
Conservatives' unrest with Trump was on display at Romney's annual donor retreat in Utah this weekend, where Meg Whitman compared Trump to Adolf Hitler.
"I think they're sitting in their cocoon, you know, away from the reality of the world," Manafort said, rejecting the Hitler comparison and calling Romney's allies "sore losers."
"You know, Romney wanted to run, chose not to. He's now attacking this past weekend all the other Republican who ran for president as well, saying they should have done a better job. Well, if he feels that way he should have run. He was a coward," he said.
Manafort downplayed the importance of fundraising, noting that Trump "spent considerably less money than most of the candidates running against him and did very well. He beat them all. So money is a component of an election and in a national race -- it's important. But it's certainly not the only one."
"I don't think he's going to be outspent," he said. "I think we're going to raise the money."
He also said he's not concerned about Clinton's advantage in field staff in battleground states, saying Trump will benefit from the Republican National Committee's infrastructure.
"We're not worried about the field staffs in the states. I mean, you get an underreported story how there were 800 people in the Clinton campaign and there are only 70 in the Trump campaign," Manafort said.
"But what that misses is that then the Republican National Committee -- for example, there are over 500 people with another 600 coming out of the battleground states who have been trained, who we're working with, who will become part of our Trump campaign," he said.