* Downbeat day on Wall Street casts pall over markets
* US presidential debate could highlight domestic, global risks
* Weaker opening seen for European bourses
* Crude oil futures retake some lost ground
By Lisa Twaronite
TOKYO, Sept 26 (Reuters) - Asian shares began the week under a cloud on Monday after losses on Wall Street, and as investors' attention turned from central banks to American politics ahead of the first U.S. presidential debate.
Half of America's likely voters will rely on the presidential debates to help them make their choice between Republican Donald Trump and Democrat Hillary Clinton in the Nov. 8 election, according to a Reuters/Ipsos poll released on Monday.
The gloomy mood was likely to continue into the European session, according to financial spreadbetters.
"A good performance from Mr. Trump could see market volatility increase, particularly if investors think there is a possibility that he could actually win," wrote Michael Hewson, chief market analyst at CMC Markets in London, who expected Britain's FTSE 100, Germany's DAX and France's CAC 40 to all open lower.
MSCI's broadest index of Asia-Pacific shares outside Japan extended early losses and was down 0.8 percent by afternoon, while Japan's Nikkei stock index ended down 1.3 percent.
Wall Street logged weekly gains but ended with solid losses on Friday. Still, the S&P 500 managed to record its best weekly performance in more than two months after the U.S. Federal Reserve held interest rates steady on Wednesday.
Monday evening's showdown between Trump and Clinton (0100 GMT in Asia on Tuesday) is the first of three debates. It could set U.S. television audience records and will also be watched around the globe.
"There won't likely be any outcome affecting the Japanese market directly, but investors are watching how the currency markets will move. Anything triggering fears for a strong yen trend is negative to stocks," said Takuya Takahashi, a strategist at Daiwa Securities.
Clinton will press Trump to provide more specifics on his policies, two top Clinton campaign aides said. Asian investors in particular will be looking for more comments from both candidates about their positions on free trade deals.
The dollar slipped 0.2 percent to 100.86 yen, moving back toward a one-month low of 100.100 touched last week.
"People are worried about the uncertainty about the election, so for a while, the yen stay around its current level, between 100 and 101 yen, that area," Harumi Taguchi, principal economist at IHS Markit in Tokyo.
The euro inched up slightly to $1.1227.