A small group of ultra-wealthy Americans pumps a lot of money into politics.

We took a look at the top 25 donors who funded American politics in 2018 and found that the list consists of, among others, an assortment of financiers, heirs, and entrepreneurs.

The contributions of some of the largest donors — such as Sheldon and Miriam Adelson, George Soros and Tom Steyer — are well known, while other donors, like Jeff Bezos, are better known for their careers.

Interestingly, one well-known political donor — Charles Koch — did not make the list. Koch and his now-deceased brother David donated $1,816,650 to Republicans through their company Koch industries in 2018, according to The Center for Responsive Politics. To make this list, their donations would have needed to top $6.5 million.

Business Insider previously reported that public affairs rank as the eighth-most popular cause that billionaires donate to. Only 12.4% of billionaires reported making donations to politics in 2018, according to Wealth-X's 2019 Billionaire Census.

Collectively, the 25 ultra high net worth individuals and couples on the list totaled a whopping $573,892,284 in political donations in 2018, data from The Center for Responsive Politics shows.

Keep reading to learn more about the country's biggest political donors, ranked in order of their donations during the 2018 election cycle by The Center for Responsive Politics. Each donor's net worth, where available, was sourced from Forbes unless otherwise specified. Their party affiliations are listed according to The Center for Responsive Politics.