Donald Trump, after long saying his self-financed campaign shielded him from special interests, is preparing to start raising large donations. He reversed course, he said in early May, to ensure his campaign has the resources to compete with Hillary Clinton.

It might seem a strange reason for a man who says he is worth $10 billion. But a close analysis of Trump’s finances shows that in terms of ready cash, he would be ill-equipped to foot the bill himself.

When his campaign began last summer, a financial disclosure Trump filed said he had between about $78 million and $232 million in cash and relatively liquid assets such as stocks and bonds.

That would go fast if Trump spent an amount close to the $721 million President Barack Obama spent in 2012 up to Election Day, or the $449 million Mitt Romney spent in the same stretch.

This would leave hundreds of millions to be made up. And Trump’s businesses don’t produce that much in a year, a Wall Street Journal analysis shows. His 2016 pretax income, according to the analysis, is likely to be about $160 million.

The result is if Mr. Trump stuck with self-financing, he likely would have faced difficult decisions over whether to sell some of his properties or borrow more money against them.

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