Gallup provides useful data on the decline of Barack Obama's job approval from November 2012 to December 2013 among various segments of the electorate. The biggest drop is among Hispanics, from 75% approval to 52%. Gallup also reports 18% and 17% declines among two groups which include many Hispanics--under $24,000 income and non-whites--and 16% drops among those calling themselves moderates and moderate Democrats.
The next biggest drop, from 61% to 46%, is among voters 18-29, the famous Millennial Generation. This is in line with other recent polling.
After the 2008 election, in which Obama won 67% among Hispanics and 66% among 18-29 voters, there were widespread predictions of permanent Democratic majorities. These were not implausible, given that Hispanics and Millennials will inevitably form a larger percentage of future electorates; if Democrats could maintain such levels of support, they would indeed be hard (though not impossible) to beat.
Note that Obama's November 2012 approval levels among these two groups were almost the same as the percentages of their votes he won in 2012 according to the exit poll, 71% among Hispanics and 60% among voters under 30. Without these big majorities, he almost certainly would not have been reelected. Polling during the campaign period indicated that Obamacare was a big positive for Obama among Hispanics and, to a lesser extent, among Millennials.
Now both groups are clearly reacting against Obamacare; their Obama approval numbers track closely their Obamacare approval numbers. There are reasons to be cautious about concluding that a decline in job approval will result in a decline in voting behavior (Dan Joseph provides reasons for such caution in a thoughtful blogpost). But there is also reason to believe that Hispanics and Millennials, the two groups in the electorate with the least experience in political observation and voting, are more volatile in their partisan preference than groups which have been observing politics and voting for years. And reason, therefore, for doubting whether those 2008 or 2012 percentages for Obama are going to be replicated in Democratic percentages for years to come. Remember that Baby Boomers were about equally divided in the 1972 election between Richard Nixon and George McGovern, which Nixon won 61%-38%, but those same Baby Boomers voted for Mitt Romney in 2012.
My conclusion is that despite high Obama percentages among Hispanics and Millennials, we are not looking at inevitable Democratic majorities for years to come. The parties are likely to remain seriously competitive as they have been over the last 20 years, in which Democrats have won four of six presidential elections but Republicans have won majorities in the House of Representatives in eight of ten congressional elections.