Politico has the behind-the-scenes story of how the executive amnesty came to be. It details the political calculations behind the president’s decision:

What really worried the White House was that opposition wasn’t limited to vulnerable moderates up for reelection in Republican-leaning states. Sen. Al Franken, a liberal from Minnesota, expressed concerns. Sen. Bill Nelson (D-Fla.), who wasn’t on the ballot, pointedly asked Obama to wait until after the election. And Sen. Angus King (I-Maine), who caucuses with Democrats, declared openly that it would be a “mistake” for the president to do anything alone, ever.
When King personally delivered that message to White House chief of staff Denis McDonough, the Obama team knew it had a problem. If an independent from Maine, a state Obama won by 15 points, couldn’t support the president’s actions on immigration, they really were in trouble. David Simas, the White House political director, asked Guy Cecil, the head of the Democratic Senatorial Campaign Committee, for polling on the immigration issue. Cecil gave him numbers from Iowa and Arkansas, showing vast numbers of voters who didn’t want Obama to ease pressure on undocumented immigrants without the agreement of Congress.
The White House realized it couldn’t put out an executive order that would get attacked by candidates of the president’s own party. By that Friday night, as Obama flew home from a NATO Summit in Wales, he began calling allies to inform them of his decision to delay action — dealing yet another setback to the immigration reform advocates and the president’s relationship with Hispanic voters.

The whole story is here.