Although he had vowed to stay in the race through the Pennsylvania primary, Rick Santorum dropped out of the Republican nominating contest Tuesday, effectively clearing the way for Mitt Romney to secure the nod.

While Santorum was still running, there was a lot of speculation as to whether Newt Gingrich's supporters would go to the former Pennsylvania senator if Gingrich dropped out, forming a more united anti-Romney front. (Polling suggests they were about evenly split on their second choice.) The opposite, apparently, is not true. Santorum's exit has provided no benefit to his two remaining challengers when it comes to winning the nomination, and just little when it comes to carrying a primary. Even when Santorum remained in the race, we had Romney with a large lead in Pennsylvania, with about an 88 percent likelihood of victory. That has since gone to nearly 100 percent likelihood, according to the political markets.

Gingrich is excited to be the last legitimate challenger to Romney, at least in his own mind, having told Justin Sink of the Hill that "there's a conservative, named Newt Gingrich, and there's Mitt Romney." But with Santorum out, Gingrich's likelihood in any given primary contest remains negligible.

In fact, it's Ron Paul who has an increased likelihood of carrying a state with Santorum out of the way. The market on Paul winning any primary (other than the disputed Maine) jumped to a 7 percent likelihood Wednesday. He has small but non-negligible chances in a few states, including Kentucky, home of his son, Sen. Rand Paul. Santorum dropping out will allow Romney to focus fewer resources on some small states with late primaries, giving Paul and his massive national organization the opportunity for a surprise win.

Follow along in real time on PredictWise for presidential election odds, where Barack Obama is currently 63.1 percent likely to defeat Romney.

David Rothschild is an economist at Yahoo! Research. He has a Ph.D. in applied economics from the Wharton School of Business at the University of Pennsylvania. Follow him on Twitter @DavMicRot and email him at thesignal@yahoo-inc.com.

Story continues

More popular Yahoo! News stories:

• Rick Santorum ends presidential campaign after conceding to Mitt Romney in phone call

• How to predict Mitt Romney's running mate

• Women boost Obama over Romney in new ABC poll

Want more? Visit The Signal, connect with us on Facebook, and follow us on Twitter. Handy with a camera? Join the Yahoo! News Election 2012 Flickr group to submit your photos of the campaign in action.