America's two major political parties are inevitably coalitions, forced by the winner-take-all Electoral College and the need of candidates in single-member congressional districts to amass 50 percent of the vote, or nearly that, to win election.
In a nation of America's cultural variety, that means holding together groups that have different priorities and conflicting positions on issues.
So coalitions don't last forever, and change composition over time. John Kennedy's Democratic coalition united white Southerners and northern Catholics. Half a century later, Republican Mitt Romney carried white Southerners and white Catholics by wide margins.
Barack Obama's Democratic Party is a top-and-bottom coalition, with affluent gentry liberals and blacks, single women, recent Hispanic immigrants and young voters -- all groups of little political heft in Kennedy's day.
Now in the sixth year of the Obama presidency, with his job approval stuck below 50 percent, there are signs of strain. And choices made earlier, when Democrats held congressional supermajorities, are starting to prove troublesome.
One choice was to not bring forward immigration legislation that would provide a path to legalization for immigrants in the country unlawfully. This was a top priority for the Hispanic Caucus, but Obama and Democratic congressional leaders chose not to advance an issue that would cost them the support of some Democrats and require Republican votes.
During the 2012 campaign, this caused Obama few problems, except for some pointed questions in a Univision interview. But the president's job approval among Hispanics plummeted 23 points in 2013, according to Gallup -- more than any other demographic group.
And it may be plummeting even more. Hispanics are more likely than average to lack health insurance, and Obamacare was supposed to help. But, with a Spanish language website non-functional for two months, few uninsured Hispanics seem to have signed up, and the latest Pew Research Center poll shows Obamacare approval among Hispanics down to 47 percent.
One issue the Obama Democrats put ahead of immigration was global warming. In June 2009, Nancy Pelosi's House passed a cap-and-trade bill. That had political costs -- it was a career-ender for many Blue Dog Democrats -- and the issue never reached the Senate floor even when there were 60 Democrats there.
But it was a top priority for green gentry liberals. Green in their concern for the environment -- and green in terms of money. This year, San Francisco hedge fund billionaire Tom Steyer pledged to spend $100 million to elect candidates opposed to the Keystone XL pipeline.