Today, Rick Santorum suspended his campaign for the presidency. As USA Today notes, the former senator dropped his bid only two weeks before the Pennsylvania primary, where polls indicated he would lose his home state to Mitt Romney.

What happened? Simply put, Santorum could not withstand the wall of Wall Street money raised by Romney. Restore Our Future, the attack-ad Super PAC affiliated with the Romney campaign, runs on big money contributions from corporate titans, including at least $20 million dollars from private equity firms and investment banks. Santorum could never compete on the airwaves. According to campaign figures compiled by the Center for Responsive Politics, the Romney Super PAC spent more on attack ads against Santorum than Santorum was even able to raise for his own campaign:

Romney’s Super PAC and its wealthy donors are not the only example of big corporations circling wagons to make Romney the Republican Party nominee. Other political groups aligned with big business, from American Crossroads to the American Petroleum Institute, moved quickly to back Romney.

The enormous spending by Romney’s Super PAC, however, underscores the impact of Citizens United, the Supreme Court decision that makes such big money spending in elections possible.