Seeing as how the decline of the Democrat Party began with the influx of the "new left" in the 60's & 70's, the best thing the 'Rats could do is encourage defections to the Greens. Unless the electorate moves in a very ugly direction, the 'Rats will never be a majority party again until they get shed of their extremists.
Fortunately for Republicans and conservatives -- but maybe unfortunately the country as a whole, considering the advantages of a bipartisan consensus in waging war and confronting enemies -- the 'Rats will have to hit bottom before seriously prosecuting reform of their party. Rebuilding the party without the radical-left will require farsighted leadership, which they don't have. It will also require that they let go of power in the near term to build stronger coalitions in the future, yet the clintonization of the party has left it obessed above all with maintaining a death grip on their power.
The result is that the 'Rats will continue their increasingly desperate attempts to hold their coalition together, despite incompatible interests among its constituent elements, and including the toxic left. Republicans will therefore be able to continue peeling off elements of their coalition like the layers of an onion (e.g. like anti-communists and traditional patriots following the nomination of McGovern and the fecklessness of Carter, and blue-collar Democrats lured away by Reagan). Consider all the great Republicans & conservatives that were driven into our arms by the 'Rats: Ronald Reagan, Jeane Kirkpatrick, Charles Krauthammer, Richard Perle, Paul Wolfowitz, the list is long and exceptional.
Some day, when their decline and failure has advanced sufficiently to be utterly and undeniably obvious, even to reality challenged 'Rats, they will change direction and begin to reform, and begin to attract Republicans disillusioned by the growing faults of a party too long in power. The two party system is pretty much unavoidable in the American system of government, and the parties will continue to ebb and flow in power, with neither ever becoming drastically weaker than the other, but I suspect the 'Rats' decline relative to this grand scheme may proceed in the near future to a truly historic degree.