Democratic Sen. Kamala Harris of California suspended her presidential campaign on Tuesday. Why? Because, she said, she did not “have the financial resources we need to continue. I’m not a billionaire. I can’t fund my own campaign.” Meanwhile, former New York Mayor Michael Bloomberg, who definitely is a billionaire, has spent at least $57 million of his own money since he jumped into the race on November 24. Harris, by contrast, raised $36 million as of her last campaign filing in October. Of that, she’d spent almost $26 million since she announced her campaign last January 21. The divergence in the fates of the two candidates can be traced back to a Supreme Court decision on the constitutionality of campaign finance law. But the case involved is not Citizens United v. Federal Election Commission, from 2010. It’s a far less famous one: Buckley v. Valeo, from 1976. The decision opened the door for billionaires — and, more generally, the ultra-rich — to spend as much as they want on their own political campaigns.

The divergence in the fates of Harris and Bloomberg can be traced back to a Supreme Court decision — not from Citizens United in 2010, but Buckley v. Valeo in 1976.

One of the main forces behind the case was a young Republican lawyer named John Bolton, later to become President Donald Trump’s national security adviser for a time. In Bolton’s memoir, he proudly states that “Everyone knew the decision in Buckley v. Valeo could determine … the future shape of American politics.” Bolton was right — and his long-ago efforts continue to bear fruit today. No Limits Watergate was, among other things, a scandal about money in politics. President Richard Nixon’s 1972 reelection campaign had accepted bribes, including $200,000 from the chairman of the board of McDonald’s in return for permission from the federal government to raise the price of their Quarter Pounder cheeseburger. (You can read about this here; it’s #21 in the bill of particulars supporting the articles of impeachment against Nixon.) Soon after Nixon resigned in 1974, Congress responded with significant amendments to the Federal Election Campaign Act. This included a new limit of $50,000 per calendar year on what presidential candidates could spend of their own money on their campaign. Adjusting for inflation, that’s about $275,000 today.

Just two years later, however, the Supreme Court struck that limit down in the Buckley case. Those running for political office could now spend any amount of their own fortune they wanted. In fact, the court stated, it could be good for the wealthy to self-fund runs for office, because “the use of personal funds reduces the candidate’s dependence on outside contributions and thereby counteracts the coercive pressures and attendant risks of abuse.” The result of the case is a presidential field where billionaires can hang on seemingly forever without batting an eye and other competitors must strain to keep up by raising funds the old fashioned way: through regular, limited donations and by getting billionaires to give to their dark-money groups. Kamala Can’t Compare Kamala Harris is quite rich. Forbes estimates the net worth of her and her husband at $6 million, putting them in the top 2 percent of U.S. households. But in 2016, the campaigns of Hillary Clinton and Trump raised $564 million and $333 million, respectively. As Harris said, she can’t fund that by herself.

Bloomberg’s $57 million in ad buys in the 11 days since he announced are already more than twice as much as all Harris’s reported expenditures.