Donald Trump said Monday night he's ready to run "a different kind of campaign" while explaining his decision to fire campaign manager Corey Lewandowski earlier in the day.

The presumptive Republican presidential nominee told Fox News' Bill O'Reilly in an interview on "The O'Reilly Factor" that Lewandowski was "a good man" who helped him run a "small, beautiful, well-unified campaign" during the primary season.

"We're going to go a different route," Trump said.

He added that he plans to ramp up campaign operations heading into the general election phase of the campaign, and that he may even have some cabinet picks in place before the Republican National Convention in July.

"We have tremendous people, we have tremendous talent," he said.

Trump told O'Reilly that he would not announce his vice presidential pick before the convention in Cleveland, but was looking for someone with "great judgment" and "in the world of politics" to balance out his experience in the business world.

When asked by O'Reilly if he would support restricting guns based on size and the amount of rounds they fire in the wake of the Orlando terror attack, Trump said the "big guns" are the kind used by "the enemy."

"I wouldn't because it's a question of protection," Trump said. "Once you start, where do you end?"

His appearance on "The O'Reilly Factor" came after a day when his campaign manager was fired amid an internal campaign power struggle with Paul Manafort, the veteran operative who since March has been amassing influence inside Trump HQ, a campaign source told Fox News.

Manafort recently telegraphed through third parties he would be gone in 48 hours if Trump didn’t oust Lewandowski, who'd run his campaign from the outset. Manafort was fed up with battling Lewandowski and let the campaign know the two of them "just couldn't get along."

From there, it became a family affair. Trump’s daughter Ivanka and her husband Jared Kushner didn’t want to lose Manafort, believing him to be the more experienced hand to guide the campaign into the general election. They convinced Trump to keep him – and the only way to do that, given Manafort’s terms, was to dump Lewandowski, the source said.

Despite the shock of Trump parting ways with Lewandowski, one of his closest and most loyal advisers, just weeks before the convention, in some corners the decision was not so surprising.

The former conservative activist played a central role in daily operations, fundraising, and Trump's search for a running mate, but Lewandowski's aggressive approach also fueled near-constant campaign infighting that complicated Trump's shift toward the general election.

Another Trump campaign official, Michael Caputo, resigned Monday afternoon over a tweet he sent out earlier in the day about Lewandowski's firing, a campaign source told Fox News.

Caputo tweeted "Ding, dong, the witch is dead" shortly after news about Lewandowski broke.  Accompanying the tweet was a photo from the "Wizard of Oz," showing the feet of the Wicked Witch of the East protruding from under a house.

Campaign spokeswoman Hope Hicks confirmed Caputo is no longer with the campaign.

Republican National Committee Chairman Reince Preibus told Fox News' Greta Van Susteren in an "On The Record" interview Monday night he thought the firing of Lewandowski was a "new direction" for the Trump campaign.

"I see a pivot and seriousness to the general election," he said.

Fox News’ John Roberts and The Associated Press contributed to this report.