Is your favorite candidate bought and paid for? Here's how to find out. 4:24 PM ET Thu, 22 Aug 2019 | 01:20

Money is the mother's milk of politics, the old saying goes, and never has that been truer than now.

With some two dozen candidates aiming to capture voters' attention, the 2020 presidential campaign is widely expected to be the most expensive in history — easily eclipsing the more than $2 billion spent in 2016.

That makes it more important than ever to understand where your candidate is getting his or her funding. Sheila Krumholz, executive director of the Washington-based Center for Responsive Politics, says voters need to take the time to research candidates' financial ties.

"It's really a fundamental element to a vibrant and healthy democracy," Krumholz told CNBC's "American Greed." "People need to trust in the integrity of our electoral system, and that includes knowing who's paying for it."

That can be a challenge in a system awash in so-called dark money from organizations like Super PACs and 501(c) organizations that are not required to disclose their donors. In 2016, Hillary Clinton was the largest beneficiary, according to the Center for Responsive Politics. The pro-Clinton Super PAC Priorities Action USA spent $132 million, three times the spending of the two largest pro-Trump Super PACs combined.

But the Trump campaign had plenty of issues of its own, including revelations in the summer of 2016 that its chairman, Paul Manafort, had received millions of dollars from pro-Russian interests in Ukraine for his lobbying work on their behalf. The news fed developing concerns about alleged Trump ties to Russia.

Manafort resigned from the Trump campaign and later became one of the highest-profile targets of special counsel Robert Mueller. He is serving a seven-year prison sentence following his conviction on multiple counts of fraud and conspiracy, including witness tampering, all related to years of undisclosed lobbying income.

Manafort became the personification of money in politics, with fancy homes, cars and clothes, including his infamous $15,000 ostrich-skin bomber jacket. But he went to great pains hiding where his wealth came from.

"Manafort was motivated primarily by two things: power and money," New York Times reporter Kenneth Vogel told "American Greed."