Supporters of Republican presidential nominee Donald Trump contributed more than $18 million in less than a day to support his bid and the Republican National Committee after he debated Hillary Clinton on Monday night, according to his campaign, a major cash infusion that officials said they plan to plow into an advertising blitz in the final stretch of the race.

Steven Mnuchin, the campaign's national finance chairman, said the total raised by Tuesday night vastly exceeded the original goal of $5 million. He credited the huge haul to Trump's performance in his first direct confrontation with his Democratic rival.

"We thought it would be something that was successful, but I think we were blown away by this," Mnuchin said. "This was all a result because Donald Trump won the debate."

About one third of the money, or roughly $6 million, was donated online, the majority of which will go directly to Trump's campaign. The rest of the funds were secured over the phone through an effort to reel in major contributions for a joint fundraising committee with the national party, which can accept as much as $549,400 per individual donor. Details of the fundraising totals were first reported by the Wall Street Journal.

The big checks were solicited by a group of 100 top party fundraisers and Trump allies who spent the day at Trump Tower in New York making calls, including New York Jets owner Woody Johnson, Los Angeles investor Elliott Broidy and Dallas investor Gentry Beach. Trump's children also took part in the effort, as did his running mate, Indiana Gov. Mike Pence.

"I can't begin to tell you the number of people who had been on the sidelines that all of a sudden came forward with six figures," Mnuchin said.

Trump has lagged far behind Clinton in fundraising, collecting $166 million to her nearly $414 million through the end of August. But Mnuchin said the campaign had already budgeted resources for a $150 million advertising blitz in the final weeks of the campaign — an effort he said it will now expand.

"Every week we will dedicate more money to advertising," he said. "We have plenty of money in the war chest to compete with her."

Still, a large share of the funds raised Tuesday will go to the RNC, which is financing a get-out-the-vote operation for the entire GOP ticket. The exact breakdown of the fundraising haul will not be apparent until the campaign and its affiliated committees file reports with the Federal Election Commission in mid-October.