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."

With so much undisclosed money sloshing around in the dark recesses of the political system, how is a voter to tell who is funding their candidates?

Krumholz said that despite all the dark money, there is still plenty of information that is disclosed, and it is enough to provide a detailed portrait of every candidate. That is particularly true at the federal level, where candidates are required to make disclosures to the Federal Election Commission.

"Voters can learn from the campaign finance information who the top donors are to any particular campaign or party committee or (political action committee); which industries or sectors of industry are the top sources of funding; where the money is coming from geographically," Krumholz said.

While the FEC maintains a searchable database on its website, the Center for Responsive Politics maintains its own website — opensecrets.org — which is designed to be more user-friendly and includes analysis of the vast amounts of FEC data.

The site also offers perspective on dark money donations, helping voters connect them to candidates and campaigns in ways that are not possible using just the raw FEC data.

"We really try to slice and dice millions of data points to present it in aggregated form in a way that people can easily absorb," Krumholz said.

But the site only covers candidates and campaigns for federal office — the presidency and Congress.

For state level races, turn to followthemoney.org, a joint project of the National Institute on Money and Politics and the Campaign Finance Institute. The site claims its database documents more than $100 billion in contributions, and more than 2 million lobbyist relationships per year.

Other sites will allow you to zero in closer to the local level, depending on where you live. For example, the Virginia Public Access Project has a database on money and politics including local elections in that state. In California, the nonprofit group MapLight includes data about state and local races on its website, as well as more information about campaign finance at the national level. Disclosure requirements vary by state and locality, but your state or local election authority is a good place to start. The federal government offers an online directory.

"Look for who the top donors are. Look at the industries and know who their top recipients are, who are the candidates that they're lavishing campaign cash on, to understand the relationships between the sources of cash and the politicians who are seeking their support for election," Krumholz said.

None of this type of research would likely have tipped voters to the kinds of conflicts Manafort was engaged in, but Krumholz said his story shows why following the money is so important.

"The Paul Manafort story is a great illustration of how important it is for disclosure to be timely and for enforcement to be real," she said. "It really is a great example of why it's important for there to be disclosure and for there to be accountability and enforcement, and for that enforcement to have enough teeth that it deters people from thumbing their noses at the disclosure rules as Paul Manafort did."

See how Paul Manafort made a fortune through influence peddling around the world, then hid his activities from the public. Catch an ALL NEW episode of "American Greed," Monday, August 26 at 10pm ET/PT only on CNBC..