This article has been corrected.

Campaign finance law -- the creaky, leaky boundary between our political process and the trillions of dollars swirling around in our economy -- still affords some insights into how political forces wage their wars. For example, outside groups spending money on the presidential race must (within certain qualifications) report how much they spent and for or against whom. It's that "for or against" that's interesting, adding a little plus-minus variable to the numbers that tells us much more than we'd get just with a dollar sign.

The problem traditionally has been that parsing those reports is time-consuming. Enter ProPublica, the non-profit media site. It put together a nice little tool compiling those "independent expenditure" reports (as they're known), allowing us to more easily dive into the numbers.

And so we can see, relatively easily, how much money was spent against the top-tier Republican candidates since the beginning of the year. And when we do so, one name stands out.

This, folks, is what "stop Trump" looked like. Between the beginning of January and the beginning of May, independent groups spent $20.4 million against Trump that was reported to the Federal Election Commission.

The big money started to come in after Nevada.

Among the biggest spenders in the FEC reports were Our Principles PAC, a group started by a former Mitt Romney campaign staffer, which spent $18 million. Club for Growth's political arm spent $11.4 million. And American Future Fund, a PAC supporting John Kasich, spent $12 million.

And, again, that number doesn't capture everything. CNN's Jake Tapper cited reporting from Kantar Media indicating that over $75.7 million was spent running anti-Trump ads on network television.

Which means that far, far more was spent trying to keep Trump from the nomination than the $38 million Trump has reported loaning himself so far.

One of those turned out to be a good investment.

Correction: ProPublica's tool limited daily independent expenditure listings to 20 in total. The article has been corrected to include all filings.