Ever since the Clinton era, the Democratic party has been fairly ruthless about identifying political liabilities and severing them. In the early 1990s you had welfare and crime, after 2004 you had gun control, and now there's immigration:
Podesta and Sharry assembled a roster of boldfaced Democratic pollsters — Stan Greenberg, Celinda Lake, Guy Molyneaux — to figure out how the party would ever get away from one of the most devastating GOP lines of attack, that a comprehensive immigration plan amounted to “amnesty” for illegals.
The results made Greenberg a convert.
His surveys of swing districts in 2006 and 2007 concluded that Democrats took a political risk by discussing immigration. Greenberg thought frustration with immigrants would spawn an environment similar to the welfare backlash in the 1990s and that Democrats needed to get tough on border security before talking about citizenship. ...
This time around, the message starts with a pledge to secure the borders and crack down on employers. It then moves to this: “It is unacceptable to have 12 million people in our country who are outside the system. We must require illegal immigrants to register for legal status, pay their taxes, learn English and pass criminal background checks to remain in the country and work toward citizenship. Those who have a criminal record or refuse to register should be deported."
To be sure, this is primarily a change in message, not a change in substance as with other issues. But it still suggests an important decision about the politics of the issue. It's a clear recognition that the position of the party's base puts it at odds with the majority.