WASHINGTON, Sep 17 (Reuters) Democratic presidential contender Barack Obama may joke about preparing for debates by riding bumper cars, but the 2008 campaign has been fairly civil so far -- and that's not necessarily good for US voters.
Conventional wisdom, and some research, has held that negative campaigning turns off voters and prompts them to stay away from the voting booth, but recent scholarship is reversing that notion, researchers say.
''Democracy itself requires negativity,'' said John Geer, a Vanderbilt University professor who studies negative political campaigning. ''We want the right to be critical of those in power.'' With the country highly polarized over issues from the Iraq war to abortion, the campaign will inevitably turn negative as the November 2008 election approaches, analysts said.
That will likely produce more attacks like the one this week in which Republican front-runner Rudy Giuliani accused leading Democrat Hillary Clinton of ''spewing political venom'' because she questioned the war assessment of the top US commander in Iraq.
But candidates have largely held their fire so far, in part because of the need to watch their spending on things like ads, the usual source of negative attacks, because of the unusually early start to next year's contest.
That may not be helpful as voters try to decide among candidates.
When Obama airs ads about his belief in ''unity instead of division,'' or a sweaty Republican Mitt Romney jogs along a tree-lined road while a narrator proclaims he has the energy to change Washington, voters learn little of substance to help them make a decision.
But when candidates clash over issues, as Romney and Giuliani have over illegal immigration, and Obama and Clinton have on foreign policy, they have to be specific, highlight differences and get their facts right or risk looking foolish.
''It turns out that negative ads are more likely to produce those outcomes than positive ads,'' Geer said, because negative attacks work if they are credible and can be documented.
NEGATIVE ADS STICK Negative details also stick in people's minds, even if they say they don't like attacks.
''Negative information in almost every context that we know about it stays with people longer, it's more memorable,'' said Ruth Ann Weaver Lariscy, a professor who specializes in political communication at the University of Georgia.
Weeks or even months later, people may not remember the source or specifics of a negative message but they will remember there was something negative, she said.
So far in the 2008 presidential race, the attacks have been few and relatively tentative.
Clinton tried to paint Obama as a novice on international affairs after he said during a Democratic debate he would meet the leaders of countries like Iran and North Korea during his first year in office.
Other presidential contenders joined the criticism, prompting the Illinois senator to quip later that he had to prepare for a debate in Iowa by riding bumper cars at the state fair.
On the Republican side, Romney has accused former New York Mayor Giuliani of running the city like a sanctuary for illegal immigrants. Giuliani fired back with his own attack on Romney's handling of similar cities in Massachusetts when he was governor.
Democrats and Republicans mostly have used their ads and online videos to polish resumes and air political positions, neither of which tends be as effective as critical ads.
''You may feel good, but it's not very substantive,'' Geer said.
Plus, while people complain about negative ads, they tend to give the content of positive ads a pass, even though they can contain just as much exaggeration.
While the race will get more negative as the elections approach and a clear pair of front-runners emerge, analysts said that won't be a bad thing.
Deborah Jordan Brooks, a professor of government at Dartmouth, said her research and others contradicted the long-held belief that negative ads depress overall voter turnout.
''There is just no longer any reason to think that there's a negative effect of negativity or incivility,'' she said.
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