Past the time
Legally there are more than sufficient grounds, especially by the standards the GOP set in the Clinton impeachment farce.
But at this stage in the game it might actually be counterproductive. An impeachment in the midst of an election year could alienate voters. Better to leave the guy in there as an embarrassment to hang on the GOP candidates. Besides there's no easy way to impeach both Bush and Chenney, and a Dick Chenney presidency is a horrifying alternative.