Post hoc ergo propter hoc
Liberals use this fallacy frequently. For example, they use the fact that Barack Hussein Obama was elected, and then the economy allegedly started to recover to show that Obama's election caused the economy to recover, which is complete nonsense. Another ridiculous example that they use is good things that supposedly happen in societies that have embraced homosexuality.
A variation holds that if X happened and then Y did not happen, X must have prevented Y. Those disputing a causal relationship sometimes refer to X as an anti-tiger rock, after the joke that a certain rock must have the magical ability to repel tigers because there are no tigers around. Liberals often justify their favored laws on the basis of what bad things have not happened, without presenting evidence or argument that those things would have happened otherwise.