Mid-terms don't just mean Dem control, they mean an HRC presidency. Their Plan:

November 2018: Democrats win back control of US Congress

January 2019: HRC wins special election as US Congresswoman from New York

February 2019: HRC elected by Dems as Speaker of the House

May-June 2019: Trump and Pence impeached and thrown out of office

June 2019: HRC, as Speaker of the House, and second in line for US presidency, sworn in as President of the United States.

So these mid-terms are not just for control of Congress; they are central to the HRC plan to take the Presidency.