The billionaire founder of TD Ameritrade will back Trump after spending millions trying to sink his campaign

AP Photo/Nati Harnik

Joe Ricketts.

Joe Ricketts, the billionaire founder of TD Ameritrade, will now back Donald Trump's bid for the presidency after months of attempting to block his nomination.

Ricketts, along with his wife, Marlene, is planning on spending $1 million in an effort to help Trump win in November, The Wall Street Journal reported.

The Journal also noted that Sheldon Adelson, the billionaire casino magnate, is planning on spending $5 million to support Trump.

The Ricketts family previously donated nearly $6 million to an anti-Trump super PAC that released ads accusing Trump of misogyny, among other things.

"The Ricketts decided they could not sit back and watch Hillary Clinton become the next president of the United States," Brian Baker, Ricketts' political adviser, told the Journal. "Even though the Ricketts supported other candidates during the primary, they believe it is time to unite behind the nominees. … This is all about helping Republicans win in the fall."

Ricketts supported Wisconsin Gov. Scott Walker during the Republican primary battle. Marlene Ricketts has also donated to super PACs supporting many of Trump's primary competitors, according to USA Today.

In February, Trump tweeted that the Ricketts' family should "be careful" after they funded ads against him, Business Insider's Pamela Engel reported at the time.

Ricketts' son, Pete, the Republican governor of Nebraska, endorsed Trump in May.