* Little change from 8.1 percent jobless rate expected
* 8.4 percent would be a blow, 7.9 would boost Obama
* Next report, for October, just before Election Day
By Patricia Zengerle
WASHINGTON, Oct 4 (Reuters) - President Barack Obama has survived months of lackluster reports on the nation's unemployment with little effect on his approval ratings or his poll lead over Republican rival Mitt Romney, but that could change on Friday.
After a subpar performance Wednesday night against a sharp Romney in their first presidential debate, Obama suddenly seems vulnerable to any bad news. That makes the Labor Department's jobs report for September, which will be released Friday morning, a potential hurdle for Obama in the presidential race.
Polls have indicated that the weak U.S. economy and the high jobless rate - 8.1 percent in August - are voters' top consideration in deciding who they will vote for on Nov. 6.
The monthly unemployment report has been watched closely throughout the campaign, but never more so than on Friday and on Nov. 2, when the October report will be released.
The September figures are to be released at 8:30 a.m. EDT (1230 GMT) on Friday. Economists polled by Reuters expect the unemployment rate to be 8.0 to 8.3 percent, with non-farm payrolls adding 113,000 new jobs, up from 96,000 in August.
Republicans have been frustrated that a series of mediocre jobs reports appear to have had little effect on Obama's campaign.
In some months, Obama has used his presidential powers to make other news that distracted from the monthly report, such as when he announced he supported gay marriage in May. At other times, campaign missteps by Romney have grabbed the spotlight.
This week, however, a bad jobs report could resonate against the president.
During Wednesday's debate, Romney hammered Obama for his handling of the economy, and reminded voters of his own record as a successful businessman.
"The dynamics have changed after last night," Potomac Strategies Group political analyst Greg Valliere wrote in a research note to clients on Thursday. "Obama is in trouble and may get another dose of bad news when the September jobs report is released tomorrow."
CRACKING 8 PERCENT
Analysts said it would take an increase of 3 or more tenths of a point in the unemployment rate - to 8.4 percent or higher - to really shake voters' faith in Obama. They noted that a drop of 2 tenths of a point - to even 7.9 percent, would be an important psychological boost.
"Something under 8 would certainly be a headline number that the White House would like to see," Valliere said.
A middling jobs report, while not a boon for Obama, probably would not have a huge impact on the Democrat's re-election prospects, Valliere said.
With less than five weeks before Election Day, most voters have decided who they will support. With the jobless rate above 8 percent for 43 months, they have also made up their minds about the economy.
"People are well aware already that the unemployment rate has hung high, that the recovery has been disappointing, but they've adjusted to all of that psychologically, and they are going to base their voting behavior to some extent on other things," said Isabel Sawhill, an economist at the Brookings Institution.
Unemployment has been at 8 percent or above since long before Obama took office in January 2009. Polls show that most Americans do not blame him for the weak economy and trust him more on other issues.
"People still believe that (Republican) President George W. Bush is more responsible for the economic condition we're in than President Obama," said Michael Podhorzer, political director of the AFL-CIO, the largest U.S. labor federation and a major Obama backer.
The economy has been improving, if slowly. Gross Domestic Product is rising, consumers have reduced debt and confidence is rising. Stock prices are also high, good news for the roughly half of Americans who own shares directly or through investment funds.
There is a twist that benefits Obama in the unemployment picture. The national unemployment rate is 8.1 percent, but it is far lower in many of the battleground states where Obama and Romney are both fighting hard for votes.
Iowa's jobless rate is 5.5 percent, New Hampshire's is 5.7, Virginia clocks in at 5.9, Ohio at 7.2 and Wisconsin at 7.5.
Even if they do not pay attention to monthly reports, relatively strong statewide numbers make it less likely that voters in those battlegrounds would be sour on the "Obama economy" and want a change.
"I don't think that voters pay nearly as much attention to (the national unemployment report) as either campaign professionals or economic professionals. Voters judge the state of the economy by when they are talking to their neighbors, did someone get a job offer or not," said Matt McDonald, of the Republican-leaning Hamilton Place Strategies consultancy, an outside consultant to Romney's campaign.
If Obama weathers the post-debate storm, and Friday's jobless report, there is one last chance for a bad - or good - unemployment report to affect the election after all.
The last jobless report of the campaign - for October - comes out on Friday, Nov. 2, just four days before Election Day.
(Editing by David Lindsey and Claudia Parsons)
Keywords: USA CAMPAIGN/UNEMPLOYMENT