the only path they have is to expand their base outside urban core. Will not happen until they start accepting pro-lifers and pro-marriage folks back in. They also will have to stop losing white voters by 20%.
If the Democrats were limited to the urban core, they would have become irrelevant decades ago. The reality is that the Democratic party relies on very strong support in the cities and close to even results in the suburbs and in fact coming second in the suburbs did not prevent Obama from wining reelection (there are a lot more people living in urban areas than in the Republican small town and rural stronghold).
As for the social issues you cited, consider that the majority of voters (even in the strongly Republican 2014) believe that majority should be legal, while the opponents and supporters of same-sex marriage are tied, with a strong trend for the supporters. It's doubtful that the Democrats would gain much from abandoning their positions on these issues and far more likely that they would lose votes.
Oh, and winning white voters by 20% sure helped President Romney
Now improving upon this result would certainly help but would require either the Democratic party nominating again candidates that can be perceived as anti-white - which can't be relied on or changing their positions on certain issues (chief among them immigration), which the Republicans might be reluctant to do.
