WASHINGTON — With support for the war in Vietnam sagging and mass protests erupting around the nation, President 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.
On Tuesday, President 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."
Even Franklin D. Roosevelt found that speeches could fall flat. He took on the U.S. Supreme Court in a 1937 national radio address after it struck down a big part of his New Deal program. He touted a plan to increase the size of the court so he could appoint more justices. The plan failed in Congress nonetheless.
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.
Democrats in particular say Obama has put them in a precarious place politically. They are demanding that he clearly articulate his vision and campaign for it.
Obama argues the U.S. must lead the international community in deterring Syria from using chemical weapons. U.S. officials say Syria's army launched a chemical attack last month near Damascus that killed more than 1,400 people.
Dan Mahaffee, director of policy for the nonpartisan Center for the Study of the Presidency and Congress, says an Obama address from the White House, despite all its limitations, "is still more than he has done so far."
Obama will make his speech at a difficult time, with a deeply war-weary American public. He will speak on the eve of the 12th anniversary of the Sept. 11, 2001, terrorist attacks, which led to the wars in Afghanistan and Iraq.
The public backed the invasion of Afghanistan, a month after the terrorist attack. And, as the second Iraq conflict loomed in 2003, the public leaned in favor of the mission. But the public consistently has opposed a strike on Syria.
On Monday, a poll conducted by the Pew Research Center and USA Today showed 63% of Americans oppose military action against Syria, and Gallup reported that support for the operation was "on track to be the lowest for any intervention Gallup has asked about in the last 36 years."
"There are not many instances in which the public was this highly skeptical," said Kathleen Hall Jamieson, a professor of political communication at the University of Pennsylvania. "This is a very difficult speech for the president."
The skepticism, she said, has many roots. The White House insists classified intelligence proves Syrian President Bashar Assad launched the alleged chemical weapons attack. But much of the public is unconvinced. An earlier Pew poll showed just half of Americans believe there is "clear evidence" a chemical attack occurred.
Still fresh in the public's mind are the many pronouncements from President George W. Bush that Iraq was hiding weapons of mass destruction. In that case, U.S. intelligence reports, which the public overwhelmingly trusted, proved false. The media, which largely reported the Iraq intelligence uncritically, have been more cautious, opting to refer to the Syrian incident as an alleged chemical weapons attack. Assad denies that his forces carried out the attack.
"Public opinion is anchored in what happened in Iraq," Jamieson said. "The president is now trying to walk in and reset what the public thinks."
Further complicating Tuesday's pitch for Obama is the amount of time he waited to make it. In previous military actions, including in Iraq, Afghanistan, Libya, Kosovo and Grenada, presidents addressed the nation, framing the issues, before Americans had much awareness of the situation. That will not be the case Tuesday.
"This debate has been going for several weeks about what to do," said David Gergen, who has served as an advisor to four U.S. presidents. "It is going to be hard for him to find something fresh to say.... Members of Congress say the calls they are getting from constituents are overwhelmingly against this. He now has to find a way to have people open their minds."
But beyond that, the White House "bully pulpit" is not nearly as powerful as many Americans — and many politicians — believe. Studies of public opinion have shown that none of the presidents hailed as "great communicators," including Roosevelt, Ronald Reagan and Bill Clinton, actually managed to change the public's view on major issues.
Indeed, in this highly polarized era, the opposite can happen: When a president takes a position on a prominent issue, members of the party out of power often grow more opposed. That appears to have happened on Syria in recent days. Pew's polling shows that among self-identified Republicans, opposition to a military strike in Syria has soared, with 70% of Republicans now saying they oppose strikes, compared with 40% a week ago.
Obama seems to be well aware of the limitations of the prime-time White House address, as he has made very few of them. There were Oval Office addresses on the Gulf of Mexico oil spill and the end of combat operations in Iraq, and statements from the East Room, where Obama discussed the killing of Osama bin Laden, America's withdrawal from Afghanistan and the budget deadlock that brought the country to the verge of default. Obama's advisors have at times been dismissive of the approach, saying the prime-time speech no longer reaches the audience it once did.
Not so long ago, when a president delivered a prime-time speech, Americans had the choice of either watching it or not watching TV. Now there are myriad options.
"The idea that a presidential address can have the impact it used to have, I think that is imaginary," said Richard Norton Smith, a presidential historian at George Mason University. "Using that medium in this climate to address something as nebulous and intrinsically unpopular as yet another Middle East military intervention? It is a mismatch."