Two rogue billionaires — one on the left, one on the right — have emerged as the biggest political spenders of the 2018 elections, defying their own parties and pouring millions of dollars into confrontational campaign tactics.

Democrat Tom Steyer $15.7 million Republican Richard Uihlein $20.0 million

Tom Steyer, a former hedge fund investor based in California, is the biggest individual donor on the Democratic side. His Republican counterpart is Richard Uihlein, an elusive packaging supplies magnate from Illinois.

Mr. Steyer is a controversial figure among Democrats. He has funded ads calling for President Trump’s removal and has chided Democrats for being too timid.

Mr. Steyer’s direct political spending goes through NextGen Climate Action, an advocacy group he controls, which finances ads and passes money to more traditional Democratic PACs. In the last two election cycles, he has spent more later in the season.

In addition to the spending reported to the Federal Election Commission, Mr. Steyer has committed $20 million to his campaign to impeach the president, which has included digital and TV ads, and billboards in prominent locations like Times Square.

Mr. Uihlein has mostly supported political committees that back hard-line conservatives. He began stepping up his federal contributions during the 2016 elections, and this year, has already spent close to the total he spent in that entire cycle.

Comparing how much they have given in previous elections Richard Uihlein Through Feb. 28 2012 2014 2016 2018 Tom Steyer Through Feb. 28 2012 No contributions to PACs 2014 2016 2018 $20 $40 $60 $80 mil. Richard Uihlein Through Feb. 28 2012 2014 2016 2018 Tom Steyer Through Feb. 28 No contributions to PACs 2012 2014 2016 2018 $20 $40 $60 $80 mil.

Long a power broker in Illinois and Wisconsin political circles, Mr. Uihlein has taken an even more aggressive political approach this year.

He has intervened in Republican primaries against incumbents, donating to a super PAC that backs an insurgent Senate candidate in Mississippi and, at the state level, bankrolling a hard-right challenger to the Republican governor of Illinois.

Tiffany Muller, president of End Citizens United, a Democratic group that supports campaign finance regulation, said Mr. Uihlein was among the most important donors on the right but has not “gotten the same attention as the Kochs or even the Mercers.”

“He supports the most extreme candidates and inflexible positions,” Ms. Muller said of Mr. Uihlein, adding: “It’s not just candidates he funds, but right-wing groups to try to influence policy.”

Much of Mr. Uihlein’s money so far this cycle has gone to the Club for Growth, a small-government committee that operates aggressively in primary elections. He has also given to a super PAC controlled by John Bolton, including a quarter-million dollars in February, shortly before Mr. Bolton was appointed national security adviser.

Where the bulk of Mr. Uihlein’s money is going Club for Growth and local affiliates Advocates limited government $7.3 million Solutions for Wisconsin Supports Kevin Nicholson for Wis. Senate $3.5 million Restoration PAC Opposes Democratic Senate incumbents $2.5 million America’s PAC Focuses on black, Hispanic and swing state voters $1.7 million Tea Party Patriots Citizens Fund Supports conservative primary candidates $1.6 million John Bolton Super PAC Suspended activity after Mr. Bolton’s appointment $850,000 Note: Donations are through Feb. 28.

David Keating, a campaign finance expert who favors fewer restrictions on political spending, said the rise of donors like Mr. Uihlein and Mr. Steyer mirrored the country’s broader political polarization.

“There are probably more donors who are ideologically driven today than there were in the past,” said Mr. Keating, a former strategist for the Club for Growth.

But Mr. Keating cautioned that the prominence of activist donors could also fade over the course of the year, once primary season is over. Many wealthy donors, he said, prefer to save their spending for the more straightforward fall contests that pit one Republican against one Democrat.

Indeed, beyond Mr. Uihlein and Mr. Steyer, a longer list of individuals who have donated the most so far shows many major givers who appear more conventional in their practices.

Top donors through Feb. 28 Rank Donor Total given Main recipients 1. Richard Uihlein Chief executive of Uline, a packaging supply company $20.0 million Affiliated with hard-line conservative candidates 2. Tom Steyer Former hedge fund investor $15.7 million A climate-related group 3. Bernard Marcus Co-founder of Home Depot $5.3 million Affiliated with Republican leadership 4. Fred Eychaner Media executive $4.0 million Affiliated with Democratic leadership 5. George Marcus Real estate developer $3.8 million Affiliated with Democratic leadership S. Donald Sussman Founder of Paloma Partners, a hedge fund $3.8 million Affiliated with Democratic leadership 7. Steven A. Cohen Hedge fund investor $3.0 million Affiliated with Republican leadership Charles G. Koch Chief executive of Koch Industries $3.0 million Affiliated with the Koch network 9. Abigail Wexner Lawyer and wife of founder of L Brands $2.5 million A veteran-related group 10. Henry Laufer Hedge fund investor $2.2 million Affiliated with Democratic leadership 11. Geoffrey H. Palmer Real estate developer $2.1 million Affiliated with Mr. Trump 12. Deborah Simon Daughter of Melvin Simon, a shopping mall developer $2.0 million Affiliated with Democratic leadership 13. Robert Mercer Former executive of Renaissance Technologies, a hedge fund $1.9 million Affiliated with hard-line conservative candidates 14. Hushang Ansary Former Iranian ambassador to the United States $1.5 million Affiliated with Republican leadership James H. Simons Founder of Renaissance Technologies, a hedge fund $1.5 million Affiliated with Democratic leadership Cynthia Simon Skjodt Daughter of Melvin Simon, a shopping mall developer $1.5 million Affiliated with Democratic leadership George Soros Hedge fund manager $1.5 million Affiliated with Democratic leadership 18. Andrew Beal Banker $1.0 million Affiliated with hard-line conservative candidates Paul L. Foster Oil executive $1.0 million Affiliated with Republican leadership Amy Goldman Fowler Author and daughter of a New York real estate investor $1.0 million Affiliated with Planned Parenthood Richard Gilliam Coal executive $1.0 million Affiliated with the Koch network Kenneth C. Griffin Chief executive of Citadel, an investment company $1.0 million Affiliated with Republican leadership Clayton Mathile Former owner of the Iams Company, a pet food maker $1.0 million Other Republican group Richard Rosenthal Philanthropist and former publishing executive $1.0 million Affiliated with Planned Parenthood Paul Singer Hedge fund manager $1.0 million Affiliated with Republican leadership Jeff Yass Co-founder of Susquehanna International Group, an investment company $1.0 million Other Republican group + Show more donors - Show fewer donors Note: All donations are from Jan. 1, 2017 through Feb. 28, 2018.

Fred Eychaner, a billionaire media executive, and George Marcus, a real estate developer, have each given about $4 million, mainly to super PACs linked to Democratic leaders in the House and Senate.

Bernard Marcus, a Home Depot co-founder, and Steven A. Cohen, a hedge fund investor, have given a combined $8.3 million, mainly to establishment-aligned Republican groups.

Matthew Rothschild, executive director of the Wisconsin Democracy Campaign, which supports stricter campaign-finance regulation, said Mr. Uihlein’s political role showed how big donors had come to overshadow traditional political parties.

“The parties are increasingly irrelevant,” Mr. Rothschild said. “Billionaires can just set up their own organizations and just dominate a political campaign.”

Mr. Uihlein has done that to a great degree in Wisconsin this year, Mr. Rothschild said, by giving $3.5 million to a super PAC supporting Kevin Nicholson, a Republican running for the Senate. Mr. Nicholson faces a contested primary against a fellow Republican who is backed by a different billionaire.

Mr. Rothschild said voters should not mistake the emergence of competing billionaires as a sign that the campaign finance system is basically stable and fair.