Once upon a time, the two parties' national conventions chose presidential nominees. Now, they are television shows that try to establish a narrative -- one that links the long-since-determined nominee's life story with the ongoing history of the nation, one that shows how this one man is perfectly positioned to lead America to a better future. The hope is that the nominee will get a bounce in the polls.

And they usually do. Gallup poll data shows that nominees got a 5 percent or better bounce from 14 of the 16 national conventions between 1976 and 2004. And that's even for nominees that in retrospect seem less than inspiring.

In 1988, Democrats presented Michael Dukakis as the son of immigrants who produced the Massachusetts miracle; Republicans presented George H.W. Bush as the pioneer who went to Texas and was now ready to take on another mission. Both got 11 percent bounces.

The biggest of all -- 30 percent -- went to Bill Clinton, "the man from Hope" in 1992, helped by Ross Perot's withdrawal on the day of his acceptance speech. The notable exceptions came in 2004, when a polarized electorate gave George W. Bush only a 4 percent bounce and John Kerry -- "reporting for duty" -- actually lost ground.

There is a difference between the two parties, however. The Democrats can usually depend on the mainstream media accepting their narratives uncritically, while the Republicans can expect them to punch holes in their storylines. In 1988, the media didn't note that Dukakis was less an earthy ethnic than a reformer in the Massachusetts Puritan tradition, but it was eager to point to the senior Bush's aristocratic Eastern background.

The narrative of this year's Democratic National Convention can be forecast with some assurance. It will emphasize Barack Obama's roots in Kansas more than Kenya or even Hawaii; it will portray him as a leader from a new generation eager to cast off the partisanship of the last decade; it will hail him as a symbol that America has risen above past prejudices and can once again stand proud in the world. His acceptance speech in Invesco Field will invite comparison with the other two Democratic nominees who spoke in stadiums, Franklin Roosevelt in Philadelphia's Franklin Field in 1936 and John Kennedy in the Los Angeles Coliseum in 1960.

An interesting question is whether mainstream media have any appetite for undermining this undeniably attractive narrative. Of "the whole Obama narrative," one reporter told The New Republic's Gabriel Sherman, "like all stories, it's not entirely true."

Obama's record of reaching across party lines is, as his own answer to Rick Warren's recent Saddleback Civil Forum showed, pretty thin. His paper trail is surprisingly thin, too. He has left no papers from his Illinois Senate days; he hasn't listed his law firm clients or provided more than one page of medical records; the papers of the Chicago Annenberg Challenge, which he chaired and in which the unrepentant terrorist Bill Ayers was heavily involved, were suddenly closed to National Review's Stanley Kurtz by the Richard J. Daley Library at the University of Illinois.

Mainstream media, with the conspicuous exception of ABC News' George Stephanopoulos, have shown little curiosity about Obama's connection with Ayers. It will also be interesting to see if there is much coverage of Obama's 2003 vote in Illinois against protecting infants born alive in attempted abortions, now that his campaign has conceded the bill was virtually identical to one that passed the U.S. Senate 98-0 in 2001.

Obama backers dismiss attempts to undermine his narrative as distractions or as racism, beyond the bounds of reasonable discourse. Most of the mainstream media tend to agree. Ayers is no more likely to appear at the convention than the disgraced John Edwards. But other media have a voice. Obama will probably get a nice bounce out of his convention. But it's not clear whether his narrative can be sustained in the weeks and months ahead.

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See Other Commentaries by Michael Barone

Views expressed in this column are those of the author, not those of Rasmussen Reports.