The casino magnate Sheldon G. Adelson told Donald J. Trump in a private meeting last week that he was willing to contribute more to help elect him than he has to any previous campaign, a sum that could exceed $100 million, according to two Republicans with direct knowledge of Mr. Adelson’s commitment.

As significant, Mr. Adelson, a billionaire based in Las Vegas, has decided that he will significantly scale back his giving to congressional Republicans and direct most of his contributions to groups dedicated to Mr. Trump’s campaign. The two Republicans familiar with Mr. Adelson’s plans spoke anonymously because they were not authorized to discuss the matter publicly.

Mr. Adelson’s pledge to Mr. Trump, the presumptive Republican presidential nominee, comes at an opportune time. Mr. Trump has relied on a mix of his own wealth and small-dollar contributions to finance his primary effort and lacks the sort of major donor network needed to sustain him in the general election. Mr. Trump has said that he may need $1 billion for the campaign but has only recently begun scheduling fund-raisers and hiring finance staff members. Many of the Republican Party’s wealthiest contributors, including the billionaire brothers Charles G. and David H. Koch, have indicated they are unlikely to give to his candidacy.

What remains unclear is how Mr. Adelson plans to contribute his money to Mr. Trump. He will give the maximum allowed to Mr. Trump’s campaign and the Republican National Committee, but to spend the amount he contemplates would require donating through a “super PAC,” able to accept unlimited donations.