If Democrats fail to get their voters out in the mid-term elections, then there's no doubt they'll lose their majority in the House and be in danger of losing their majority in the Senate as well. Democrats are either tied or down among Independents with Republicans so their best shot at retaining control in both chambers in Congress is to energize their sleepy base, especially the social Democrats, and convince them that Republican control of either the House or Senate would be a disaster for the Progressive movement and would almost erase everything that Democrats have fought to accomplish the last 4 years.
Democrats are certain to lose seats in both the House and Senate in the mid-terms and they already know that the question is how many, but can Democrats hold Republican gains in the House to 30 or less where they would still have a 10-seat majority? Or will Democrats lose more then 30 in the House and come close to losing their House majority? Or will Republicans pick up more than 40 seats in the House and have a clear majority like the Democrats? So the only victory Democrats can hope for in November is holding on to their majorities in both the House and Senate, which is why getting out their vote is critical for them to remain in control of Congress.