Warn all they want if he wants to use his pen he will.
Check it out:

Earlier this month White House Press Secretary Josh Earnest said President Obama was “very interested” in the possibility of raising taxes through executive fiat—a move GOP lawmakers warn would be a grave mistake.

Rep. Paul Ryan, chairman of the House Ways and Means Committee, and Sen. Orrin Hatch, chairman of the Senate Finance Committee, said that doing so would be “a mistake, both constitutionally and politically.”

Hatch, of Utah, and Ryan, of Wisconsin, sent the letter to [Treasury Secretary Jack] Lew after the White House signaled Obama is interested in eliminating some tax breaks through executive action, particularly those that benefit large corporations and the wealthy.