Actually according to US voting patterns Democrats should make significant gains, simply given "generic Republican" in office with no major issues. In most mid-terms the President's party loses ground.
The question is how large, and whether it's enough to turn balances of power around. The simple fact of Party X holding the White House produces its own backlash in Party Y's favor, barring some significant influence to the contrary such as 1934 when FDR's Democrats in the midst of combating the Depression for which Hoover was blamed, gained seats instead of lost them, but that's the rare exception.
This President so far is trending in the opposite direction though so far.
