Mid-term elections tend to favor the party out of power. If the Democrats hope to keep a hold on both chambers, and they need to if they want to keep pushing their agenda, then Democratic voters need to turn out, in big numbers.
As it stands now, there is really only about a year left to get things done. By this time next year every Representative and 1/3 of the Senators will be knee deep in reelection campaigning. Those who are in purple districts or states will have to walk a fine line, lest they upset potential voters.
Unfortunately, it is unlikely we will see any major reforms this year. Health care reform (don't even dream about single-payer) is probably a non-starter, although it could still be done at a State level. $15 per hour minimum wage SHOULD be doable but, again, that could be handled at the State level. Now is the time to fill vacant judicial posts, get cabinet positions filled, and set the agenda. Get ahead of the Republicans on the PR level. Be the party of progress and make them play defense. Make them spend the political capital fighting just to not fall further behind.