TOKYO, July 14 (Reuters) - Japan's Nikkei share gauge

slid for a third-straight session on Monday as concerns about an

upcoming domestic election and unresolved trade talks with the

United States weighed on investor sentiment.

The Nikkei 225 Index closed 0.3% lower, while the

broader Topix gauge ended flat.

Japanese Prime Minister Shigeru Ishiba has said Japan would

continue tariff negotiations with the U.S. after U.S. President

Donald Trump last week raised tariffs on Japanese imports to 25%

starting August 1.

However, these talks could face challenges, as prospects of

Ishiba's ruling coalition retaining its majority in the upper

house after a vote on July 20 are dimming.

"If the ruling party were to lose its majority in the upper

house, there is a risk that trade negotiations with the U.S.

will be delayed, and market concerns about fiscal expansion will

increase," said Nomura strategist Fumika Shimizu.

"I think there is a great possibility that the Japanese

stock market will be affected by these developments."

There were 106 advancers on the Nikkei index against 119

decliners. The largest losers by percentage in the gauge were

online retailer Mercari ( MRCIF ), down 5.4%, followed by

videogame maker Nexon ( NEXOF ), which slid 4.2%.

The largest gainers in the index were major manufacturers

Mitsubishi Heavy Industries Ltd ( MHVYF ) and IHI, both

surging more than 3.6%.

Real trade negotiations are likely to be compressed in the

period after Japan's upper house election and Trump's latest

deadline of August 1, said Weston Nakamura, a markets analyst

who publishes the Across the Spread newsletter.

Even so, Japan and South Korea are looking comparatively

better after subsequent tariff announcements by Trump, such as

50% against Brazil, he said.

"So, this Monday, Japan and Korea's situation doesn't look

so bad anymore," Nakamura said. "They still remain the

relatively favoured nations."