Consider this year's election. The liberal Democrats did not return to power because of this or that domestic policy idea or because, more generally, they had conducted a sober reassessment of liberal dogma following prior setbacks. They returned to power, without having revised much of anything, because the electorate was sick of the Republican administration. This scenario is the rule in presidential politics, not the exception.This is a generalization and I'm sure that many voters voted for some specific proposal that Obama made, especially the dollar amounts he promised. Never the less a lot of people who voted for Bush switched to Obama. It is silly to say that these swing voters suddenly went from center-right to hard left. They voted for the other side because the felt that the Republicans screwed things up.
That doesn't mean that the Democrats can't try changing things. That is their real mandate - change. Specifically, change that you can believe in.
The Democrats' mandate is to make the country run better. How they do it is left to them but the voters are watching. If they succeed and Obama is reelected then they have a mandate to continue. Reagan won in 1980 because things got worse under Carter. By 1984 things were noticeably better and Reagan won vindication in that election (plus his vice-president was elected by about the same margin the Obama got). The voters judged Reagan and voted in his favor.
All of this makes it east for Obama and the Democrats to overreach. They want to reshape society but their mandate is really a lot more specific - make things better. If they manage to bring peace and prosperity then they will be forgiven for anything else they do along the way.
They do have a shot at this. The economic news is scary enough to let them push through some drastic measures. Opportunities like this don't happen often. Economic disasters allowed FDR and Reagan chances to make major changes. LBJ used the shock of JFK's assassination to push through a radical agenda as a memorial to JFK.
At the same time, the measures that Obama and the Democrats are planning are likely to hurt the economy. If the economy fails to improve then the Republicans will have a mandate - to reverse Obamanomics.
Unfortunately, this means that Republicans are not in control of their own future. They need to have something to offer but they will not have the chance until the Democrats stumble.