Republicans have no ideas left, at least no ideas that have not been tested and proven to be failures again, and again, and again

The economy didn't just crash under a Republican president, it crashed under Republican policies. 

It crashed with low taxes. It crashed with deregulated markets. It crashed with huge restrictions on union activity. It crashed with massive cuts in environmental regulations. It crashed with lowered trade barriers. It crashed with big fat Pentagon spending

For the same reason that they have to maintain that global warming is the creation of a conspiracy of scientists, and that evolution is a conspiracy of other scientists, and that gay marriage is a threat to "traditional" marriage. 

They have to lie about the threat of illegal immigrants. 

Lie about the state of the national debt.

Lie about the effects of the Affordable Care Act,

Lie about Social Security, Medicare and Medicaid.

They have to lie, because lies are all they have left.

It's who they are