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“This distrust of government and politicians is unfolding [just] as a full-blown crisis of legitimacy sidelines Democrats and liberalism,” Mr. Greenberg wrote, noting that only 25 percent of Americans are optimistic about the government. “A crisis of government legitimacy is a crisis of liberalism. It doesn’t hurt Republicans. If government is seen as useless, what is the point of electing Democrats who aim to use government to advance some public end?”

As the 2012 race intensifies, Mr. Obama is moderating his campaign rhetoric by asking supporters to take a more historical view of his presidency. At the event in New York two weeks ago, the president talked about the theme he planned to strike in a now-postponed speech at the dedication of the monument to Martin Luther King Jr. on the Mall.

“Now that King has his own memorial on the Mall, I think that we forget when he was alive there was nobody who was more vilified, nobody who was more controversial, nobody who was more despairing at times,” Mr. Obama said. “There was a decade that followed the great successes of Birmingham and Selma in which he was just struggling, fighting the good fight, and scorned, and many folks [were] angry.

“But what he understood, what kept him going, was that the arc of moral universe is long but it bends towards justice,” the president said. ” But it doesn’t bend on its own. It bends because all of us are putting our hand on the arc and we are bending it in that direction.”

“And,” he added, “it takes time.”