The midterm elections are pitting billionaire against billionaire.

On the left, wealthy donors, such as George Soros, Thomas Steyer and Michael Bloomberg are digging into their wallets to try and save the Democratic majority in the Senate.

Conservatives, such Sheldon Adelson and Charles and David Koch, meanwhile, are trying to tilt the balance in favor of the GOP.

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Here are the billionaire players to keep an eye on as the midterm campaign hits the home stretch.

Sheldon Adelson (estimated net worth of nearly $32 billion)

The Las Vegas casino magnate is pulling out all the stops to help the GOP take back the Senate.

The billionaire reportedly plans to spend $100 million helping Republicans in tight Senate races. He recently gave a big chunk of that money, $10 million, to Karl Rove’s Crossroads GPS.

He has taken a particular interest in the campaigns of Sen. Lindsey Graham Lindsey Olin GrahamMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Momentum growing among Republicans for Supreme Court vote before Election Day MORE (R-S.C.) and Senate candidates Reps. Tom Cotton Tom Bryant CottonBattle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates Chris Wallace presses Cotton on 'any hypocrisy' between comments on Supreme Court vacancy in 2016 and today MORE (R-Ark.) and Cory Gardner Cory Scott GardnerMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 MORE (R-Colo.).

Adelson is publicly opposed to online gambling and is putting pressure on Washington to ban it.

Charles and David Koch (a combined estimated net worth of $85 billion)

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Owners of one of the nation's largest private companies, the Koch brothers have become one of the leading financial backers of the GOP — and a top target of the left.

The brothers are reportedly planning to spend $290 million this election cycle to back Republicans in midterm races and conservative groups who are sympathetic to their cause.

Their latest effort was organizing the conservative super-PAC Freedom Partners Action Fund. The two industrialists are also financial backers of conservative groups like Americans for Prosperity and American Energy Alliance.

Labeled “climate deniers” by the left, the Kochs are skeptical of the Environmental Protection Agency’s new climate rules and strong opponents of ObamaCare.

Tom Steyer (estimated net worth of $1.6 billion)

Steyer is following in the footsteps of other billionaire donors by tapping his fortune to try and influence the elections — in his case, by backing candidates who support action on climate change.

He has pledged to spend $100 million during the midterm campaign to help Democrats hold the Senate and has promised to spend half of that from his own pocket.

The recipients of Steyer's donations this election cycle include the Senate Majority PAC, NextGen Climate Action, CE Action Committee and the Democratic Congressional Campaign Committee.

Michael Bloomberg (estimated net worth of more than $34 billion)

The former New York City mayor has vowed to spend $50 million this election cycle, with some of it backing Democrats who support gun control.

Bloomberg is a staunch advocate of firmer restrictions on firearms, and is endorsing more than 100 candidates in federal and state elections who support his point of view.

His group, Everytown for Gun Safety, is positioning itself as a counterweight to the National Rifle Association, with the 2014 election serving as the first test of its influence.

George Soros (estimated net worth of $24 billion)

Soros, a hedge fund manager dubbed “the man who broke the Bank of England,” is also looking to break the GOP.

He has cut checks for the Democratic Senatorial Campaign Committee and the Democratic Congressional Campaign Committee, as the party looks to maintain control of the upper chamber.

Soros was also a big supporter of President Obama in 2012 and has donated to the super-PAC Ready for Hillary, which is laying the groundwork for a potential 2016 run.

Dick DeVos (estimated net worth of about $5 billion)

DeVos is one of the top conservative political donors in the country, prompting some on the left to compare his family to the Koch brothers.

The Michigan entrepreneur with family ties to Amway is a proponent of the growing right-to-work movement, which argues employees should not be forced to join a union as a condition of employment

The DeVos family is a big supporter of the conservative New Republican PAC.

Paul Singer (estimated net worth of nearly $2 billion)

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The billionaire hedge fund manager is one of the most prominent gay rights supporters in the Republican Party and a strong advocate of free-market policies.

Singer steers millions of dollars to LGBT groups like Americans for Workplace Opportunity and American Unity PAC.

He also opposes financial regulations, such as the ones from the Dodd-Frank reform law, and is a top supporter of conservative groups such as Club for Growth and American Crossroads.

Mark Zuckerberg (estimated net worth nearly $35 billion)

The Facebook billionaire has his eyes set on immigration reform.

Zuckerberg formed the group FWD.us to back both Democrats and Republicans who support immigration reform, which he and other tech CEOs hope will help fill the worker shortage in Silicon Valley.

Last year, Zuckerberg drew backlash from liberals, after his group criticized ObamaCare and praised candidates for supporting the Keystone XL oil pipeline.

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Sean Parker (estimated net worth of more than $3 billion)

The Silicon Valley billionaire, known for his roles at Napster and Facebook, is beginning to dabble in politics.

Parker says he is not so much a Democrat or Republican donor, casting himself a nonpartisan observer who wants to contribute to the campaigns of politicians on both sides of the aisle who will work together.

Like Zuckerberg, Parker is a big proponent of immigration reform.

— This story was updated on Sept. 29 at 3:58 p.m.