Only with massive fraud will they flip that many seats. Ironically, Pres. Trump should have strict voter security in place to “keep Russians from meddling with the 2018 election,” but then Democrats will whine that this creates “voter suppression” among their cheating base.
It’s pretty common for the party holding the White House to lose seats in the midterm. GOP can only lose 22 and still have the majority. Pennsylvania redistricting costs 5 already. California resignations in purple districts probably another 4-5. House flipping to the Dems is not a radical call at all.
At that point the Democrats can subpoena Trump’s past tax returns.