Obama in 2012 vs Romney compared to Obama in 2008 vs. McCain may have had less support from these groups, that does not mean that the overall tendency for these groups to lean Democratic has lessened.
The Obama 'drop-off' is partly because the historic nature of his candidacy led to a spike of support from several different groups in 2008. It was also in part that Romney, in my opinion, was a stronger candidate than McCain, and Obama was weaker than his previous campaign.
So let's take a look at overall Democratic support. The following numbers show those who lean democratic vs. republican for different groups from 2004-2014. Data from here: http://www.people-press.org/2015/04/07/party-ident...
Women: 50/38 (2004), 54/32 (2008), 51/36 (2012), 52/36 (2014)
Blacks: 80/9 (2004), 81/8 (2008), 84/9 (2012), 80/11 (2014)
Hispanics: 50/23 (2006), 64/23 (2008), 57/24 (2012), 56/26 (2014)
Asians: no data (2004), 57/25 (2008), 59/27 (2012), 65/23 (2014)
Millenials: 50/37 (2004), 55/30 (2008), 53/33 (2012), 51/35 (2014)
So there was that spike in 2008 that makes it seem like support fell off in 2012. The reality is that the tendency to lean Democratic for all of those groups is the same or higher then it was in 2004.
Even if the Republicans were serious about winning and serious about actually governing this country, I don't think they have a chance. They just don't have the numbers and for some reason seem oblivious to the fact that popularity contests require popularity.
But no party that has Trump out in front is serious about winning or governing. At this point they are just the punchline of a really bad joke.