Evan Halper / Tribune Washington Bureau

WASHINGTON — With support for the war in Vietnam sagging and mass protests erupting around the nation, President Richard Nixon invited cameras into the Oval Office in November 1969 and spoke directly to Americans.

Seated behind a desk, reading from a prepared text, Nixon explained why an immediate withdrawal would be a blow to freedom and democracy, outlined a plan “to end the war in a way that we could win the peace” and promised to turn over much of the fighting to Vietnamese troops.

Playing to mainstream America’s patriotism and its skepticism of the counterculture, he concluded, “And so tonight — to you, the great silent majority of my fellow Americans — I ask for your support.”

The speech changed public opinion about Nixon — his approval ratings soared to the highest point of his first term. But opinion about Vietnam changed only very slightly, and even that shift proved a momentary blip, erased within weeks.

Tonight, President Barack Obama plans a nationally televised address from the White House on the use of military force — a proposed strike against Syria — but anyone expecting to see a big shift in public opinion probably will be disappointed. Nixon’s experience was the rule, not the exception: Presidential speeches seldom, if ever, shift how the public views major issues.

White House “speeches are very limited in their ability to sway the public,” said Julian E. Zelizer, a professor of history and public affairs at Princeton University. “This is not something presidents have had a lot of luck doing.”

Obama’s speech could help the president even if it doesn’t sway public opinion. Lawmakers who support missile strikes against Syria have implored Obama to do more to make his case publicly. A White House speech could reassure them that he has taken political “ownership” of the issue.

President’s address

Obama is scheduled to address the nation from the East Room of the White House at 6 p.m.