Ezra Klein wrote an interesting piece in The New Yorker, "The Unpersuaded," about the work of George Edwards, the director of the Center for Presidential Studies, at Texas A. & M. University, outlining a strong argument that presidents--even those considered good communicators--have far less power to persuade with public speeches than many Americans realize.
Consider the following about presidents Clinton, Reagan, and Franklin Roosevelt:
Between his first inauguration, in January, 1993, and his first midterm election, in November, 1994, [Clinton] travelled to nearly two hundred cities and towns, and made more than two hundred appearances, to sell his Presidency, his legislative initiatives (notably his health-care bill), and his party. But his poll numbers fell, the health-care bill failed, and, in the next election, the Republicans took control of the House of Representatives for the first time in more than forty years. Yet Clinton never gave up on the idea that all he needed was a few more speeches, or a slightly better message. “I’ve got to . . . spend more time communicating with the American people,” the President said in a 1994 interview. Edwards notes, “It seems never to have occurred to him or his staff that his basic strategy may have been inherently flawed.”
Reagan succeeded in passing major provisions of his agenda, such as the 1981 tax cuts, but, Edwards wrote, “surveys of public opinion have found that support for regulatory programs and spending on health care, welfare, urban problems, education, environmental protection and aid to minorities”—all programs that the President opposed—“increased rather than decreased during Reagan’s tenure.” Meanwhile, “support for increased defense expenditures was decidedly lower at the end of his administration than at the beginning.” In other words, people were less persuaded by Reagan when he left office than they were when he took office.
[P]olitical scientists Matthew Baum and Samuel Kernell...found that [FDR's fireside chats] fostered “less than a 1 percentage point increase” in his approval rating. His more traditional speeches didn’t do any better. He was unable to persuade Americans to enter the Second World War, for example, until Pearl Harbor.
In fact, Edwards' evidence suggest that many presidents achieve their policy goals most efficiently without publicly advocating for them. For a president to publicly address a policy goal, according to Edwards, is often to solidify partisan opposition against it. But it can also strengthen support among the president's own party. Presidents' public persuasion attempts often have a politicizing effect--whether they like it or not.
“Barack Obama is only the latest in a long line of presidents who have not been able to transform the political landscape through their efforts at persuasion. When he succeeded in achieving major change, it was by mobilizing those predisposed to support him and driving legislation through Congress on a party-line vote.”
Of course, this is not to say that presidential attempts to persuade cannot effect the rhetorical landscape. Jeffery L. Bineham, a rhetoric professor at St. Cloud State University, notes in a letter to the editor of The New Yorker that "death tax," "wars" on poverty, drugs, terror, and mottos like "government is not the solution but the problem," are all examples of presidential speech entering the political lexicon.