Billionaire businessman Donald Trump’s presidential run has reached a crossroads, and the sign in front of him says: “Show me the money.”

Now that he’s the presumptive Republican presidential nominee, Mr. Trump doesn’t have enough time to build the national donor network he will need, and he will be forced to keep self-funding his campaign, a veteran GOP moneyman warned Wednesday.

“He’s going to have to write a pretty big check for himself if he wants to be president,” said Fred Malek, the finance chair of the Republican Governors Association.

The real estate tycoon has gotten off relatively cheap so far, spending about $47 million on his campaign operation while getting his message out to voters with massive amounts of free airtime on TV and radio.

However, the national campaign is about to get much more expensive, especially as he goes up against likely Democratic nominee Hillary Clinton, who controls one of the most powerful fundraising machines in the history of American politics.

The Clinton campaign has raised $186 million as of the last report to the Federal Election Committee in April. She’s also backed up by several well-funded political action committees.

In 2012, GOP nominee Mitt Romney raised more than $235 million in the final six months of the race, in which more than $900 million was spent on the Republican side.

Mr. Trump has prided himself on self-funding his run in the GOP primary, boasting that it makes him immune to the influence of special interests. But he has started to make noises about getting into the fundraising game as he heads into a general election contest that could cost each campaign $1 billion, including spending by political parties and outside groups.

The problem for Mr. Trump is that he doesn’t have a lot of options for filling a war chest at this late stage of the race.

He can get a super PAC to back his cause. He can fundraise jointly with the Republican National Committee, a move that appears to be in the works. He can hire an expert fundraising team. But it probably won’t be enough.

“He’s going to have a spending gap,” said Mr. Malek. “I don’t think he can raise as much money as traditional presidential candidates have because of the lack of time. It takes a while to put together a group like that.”

Mr. Trump also could have difficulty convincing people to open their wallets because he is perceived as immensely wealthy.

“I’ve seen this happen in the past. They are like, ‘Why do I have to send money to Donald Trump, why do I have to send money to Steve Forbes?’ Especially when you are Donald Trump and you’ve been telling everyone how wealthy you are since the day you announced,” said a GOP campaign consultant speaking privately.

Mr. Forbes, the millionaire publishing executive, self-funded most of his unsuccessful campaigns for the Republican presidential nomination in 1996 and 2000.

Mr. Trump claims to have a $10 billion fortune. But much of his wealth is tied up in real estate that he would have to sell or take loans against to pay off the next six months of campaigning.

The assets include about $2.1 billion in New York City properties and about $366 million in golf clubs around the country and the world, according to estimates by Forbes magazine.

In a pinch, Mr. Trump could sell off Trump Tower in New York for about $630 million or the Trump National Golf Club in Los Angeles for about $113 million, though he would pocket slightly less in both deals after paying off debts and other costs, according to Forbes.

Mr. Trump probably wouldn’t want to part with those marquee properties.

“Do I want to sell a couple of buildings and self-fund? I don’t know that I want to do that necessarily,” Mr. Trump said on MSNBC’s “Morning Joe” program. “I do love self-funding. I don’t want anything for myself. But we do need money for the party.”

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