Snopes, not the Onion. And a fair point, but if you scroll through their articles on mass shootings you'll find every last one of them has some reason it "doesn't count" when the shooter is a democrat while, for instance, Ted Cruz being a Republican is given a straight-up "True" rating.
Other times Snopes actively obfuscates when the evidence isn't with them, as here:
The claim that vastly more mass shootings have taken place during Obama's administration than that of any other President is a false one based on double standards.
www.snopes.com
FALSE: The evidence they used changed standards due to the FBI! Some of those are mass killings or mass homocides instead of mass shootings. Obama only had 27 mass shootings compared to the other presidents, and only 11 of them had 8 or more victims!
Me: Wait, the list of other presidents....
They're all still significantly lower than 27 mass murders and 11 with 8 or more deaths... it was true all along.