Two billionaires are planning to open their pocketbooks for Republican nominee Donald Trump in the final six weeks of the election, as the businessman continues to lag behind Democrat Hillary Clinton in fundraising.

Billionaire casino owner Sheldon Adelson and his wife, Miriam, are planning to spend $45 million to boost Republicans in the next two months, including at least $5 million to help Mr. Trump, according to a person familiar with the plans.

And...