Foster Friess took the fortune built through his stock-picking firm and turned it into a force in Republican politics.

When former senator Rick Santorum ran for the Republican presidential nomination for the 2012 election on a shoestring budget, the multimillionaire stepped in as a primary financier. Mr. Friess’s money supercharged Mr. Santorum’s campaign and propelled the presidential hopeful to a surprise win in the Iowa caucus.

Mr. Friess became one of the most sought-after GOP mega donors and a supporter of Donald Trump’s presidential run. He channeled his money to push for conservative fiscal policies, donating more than $500 million to charitable and political causes over his lifetime, according to Foster’s Outriders, his civic nonprofit foundation.

He died on May 27 at age 81. The cause was myelodysplastic syndrome, a disorder of the blood cells and bone marrow, a family spokesman said.