Washington (CNN) A small cluster of Democrats, led by former New York Mayor Michael Bloomberg, donated a combined $226 million to outside groups in the 2018 midterms for Congress, or nearly $1 out of every $5 spent by outside groups in those elections.

The spending helped Democrats win back the House. Now, some of these mega-donors -- including the billionaire pro-impeachment activist Tom Steyer -- are poised to help shape the 2020 races for the presidency and Congress.

Here's a glance at the Democrats' five biggest donors and the amounts they donated to outside groups at the federal level in the 2018 election cycle, according to the nonpartisan Center for Responsive Politics:

Michael Bloomberg

Amount donated to outside groups in 2018: $95 million

The former New York City mayor -- first elected as a Republican -- joined the Democratic Party in 2018 to do battle against President Donald Trump and his policies. The billionaire decided against his own presidential bid in 2020, but the millions he dedicated to Democrats in 2018 helped the party seize control of the House in the midterms.

In an op-ed this year announcing his decision against seeking the presidency, Bloomberg said he plans to his plow money and energy into the issues that have long animated him -- including expanding renewable energy as a way to combat climate change, promoting gun safety and education reform.

But Bloomberg, whose wealth is estimated at more than $54 billion by Forbes, also is expected invest heavily to help build the organizational and technical infrastructure to aid the Democrats' eventual White House nominee.

His aides did not respond to interview requests. Bloomberg himself is speaking out often about Trump's policies, penning an op-ed this week that called Trump's plans to impose a 5% tariffs on goods from Mexico over immigration "radical and disturbing." Bloomberg called on Congress to rein in Trump's power to set trade policy.

Tom Steyer

Amount donated: $72.4 million

Steyer, a billionaire hedge-fund founder, is an unabashed progressive who made his first big splash in national Democratic politics in 2014 with a multimillion-dollar effort to put climate change at the center of the political debate.

In recent years, he's also worked to make himself the face of the anti-Trump resistance with a barrage of television commercials calling for Trump's impeachment.

Along the way, he's built a massive political organization.

More than eight million people, for instance, have signed on to his impeachment campaign, giving Steyer and his aides a formidable email list of like-minded voters. And his political groups employed 750 during the 2018 midterms with operations on 420 college campuses to drive youth turnout.

"We were the largest grassroots organization in the United States in 2018," Steyer recently told CNN. We'll definitely be doing that work in 2020."

Steyer, who opted against his own presidential bid in 2020, said climate change and impeachment likely will be big factors in determining which Democratic candidates receive his support in 2020. He called impeachment "the major issue facing the United States of America on a real-time basis."

"It's going to be very hard to understand how we could support somebody who tries to duck" the impeachment question, Steyer said in the interview.

S. Donald Sussman

Amount donated: $22.4 million

The billionaire hedge-fund manager, who rarely grants interviews, has been one of the Democratic Party's biggest financial backers in recent elections. In the midterms, Susman donated nearly $8.5 million to Priorities USA Action, the primary Democratic outside group that's working to build the party's digital infrastructure to battle Trump.

Sussman, the founder of Connecticut-based Paloma Partners, also gave nearly $10 million to the main party super PACs working to elect Democrats to the House and Senate.

One of his goals, Sussman says: helping elect candidates who will get big money out of politics.

"It's very odd to be giving millions when your objective is to actually get the money out of politics," Susman told The Washington Post in a rare 2016 interview . "I am a very strong supporter of publicly financed campaigns."

He did not respond to an interview request.

James and Marilyn Simons

Amount donated: $20.1 million

The couple has supported some of the biggest outside groups in Democratic politics during the midterms.

James Simons, for instance, donated $10 million to a single group, the House Majority PAC, which worked to help the party win control of the chamber in 2018. He's an MIT-educated mathematician, who founded Renaissance Technologies, a New York hedge fund.

Simons -- worth more than $21 billion, according to Forbes -- stepped away from day-to-day management of the fund in 2009. Their aides did not respond to interview requests.

George Soros

Amount donated: $16.7 million

The hedge-fund tycoon is among the best-known Democratic donors in the country. He broke into national politics in 2004 when he spent millions of his fortune in a failed attempt to deny President George W. Bush a second term.

Around the world, the Hungarian-born billionaire is known for sending billions of his own money into liberal, Democratic causes. He's vilified on both sides of the Atlantic by conservative politicians.

One baseless claim that circulated in the run-up to last year's midterm elections sought link Soros to a migrant caravan making its way to the United States' southern border.

Soros donated nearly $10 million in the 2016 campaign to a super PAC supporting Democratic presidential nominee Hillary Clinton. In the 2018 midterms, he donated to an array of major Democratic groups, including $5 million to Win Justice, a collaboration between Planned Parenthood Voters, the Service Employees International Union and other liberal groups that mobilized people of color, women and young people to vote in the midterms.

"We are likely sitting out the primary," his political adviser Michael Vachon told CNN in an email earlier this year, when asked about the 2020 White House race.