It is helpful to Democrats to have such a ridiculous antagonist in the White House, but if they think Trump's failures will translate directly into Democrat gains, they need to think again.
Based on Democrat behavior, they still haven't learned the instructive lessons delivered in the recent presidential election.
Greenwald provides a good explanation.
https://theintercept.com/2017/02/09/tom-perez-apologizes-for-telling-the-truth-showing-why-democrats-flaws-urgently-need-attention/