EXCLUSIVE: Billionaire Sheldon Adelson just committed $25 million to an anti-Hillary Clinton Super PAC to try and help tilt the presidential race and down ballot House and Senate races to Republicans, Fox News learned Monday. There are indications the casino magnate will pony up even more by the end of the week.

Two senior Republican sources familiar with the donation described it to Fox News as a “massive” amount of money to be spent during the final week of the presidential race and a sign that Adelson, who has largely been on the sidelines after initially suggesting he would give $100 million to help Donald Trump, is now going all in on the Republican nominee.

The money will benefit Future 45, a Super PAC launched by the Ricketts family in Chicago, founders of TD Ameritrade. The group has recently been running a television ad in battlegrounds comparing Clinton to Richard Nixon, calling her a “secretive, paranoid politician who destroyed 30,000 pieces of evidence.”

The Republican sources said the contribution was made in the last few days as the FBI re-ignited its investigation of Clinton’s email server, and could spark other big GOP donors – who may have thought the Democratic nominee was coasting to victory just days ago -- to step up their support of Trump in the final days.

Most importantly, one source familiar with the Adelson contribution revealed he is considering pouring as much as another $25 million dollars into the race before Election Day on November 8.

The first major contribution came as Trump visited Adelson’s Venetian resort and casino in Las Vegas for a rally with supporters on Sunday. Trump praised Adelson and his wife, Miriam, without any reference to the money that went to the Super PAC.

“I’d like to thank the owner of this great hotel, and his incredible wife – she’s an incredible woman – Sheldon Adelson,” Trump said to cheers. “Really incredible people and they’ve been so supportive and we appreciate it.”

The move represents a startling turnaround for the Ricketts and Adelson.The Ricketts initially spent millions trying to stop Trump from getting the Republican nomination, after backing Wisconsin Gov. Scott Walker in the GOP primaries.