* Nikkei joins global sell-off, falls 1 percent.
* Wall Street logs losses as U.S. political woes rattle investors
* Insurance, banks, securities shares lead losses
TOKYO, Aug 18 (Reuters) - Japan's Nikkei share average skidded to three-month lows on Friday, pressured by losses on Wall Street and a weaker dollar as doubts grow over whether President Donald Trump will be able to push through policies to boost U.S. economic growth.
The Nikkei was down 1 percent at 19,501.53 points by the end of morning trading, putting it on track to lose 1.2 percent for the week.
U.S. stocks sold off on Thursday, with the S&P 500 logging its biggest daily percentage drop in three months, as escalating worries about the Trump administration's ability to deliver on its economic promises rattled investors.
An attack in Spain added to the risk-averse move, after a van mowed through crowds of tourists in Barcelona and killed at least 13 people, with Islamic State claiming responsibility.
Domestic concerns weighed as well. While Prime Minister Shinzo Abe's support rose this month after he reshuffled his cabinet, investors continue to gauge the impact of his support levels on Japan's currency.
"The signature trade on the back of Abenomics is shorting yen, and therefore, the fragility of the government is giving more reason for the yen to appreciate," said Kyoya Okazawa, head of institutional clients, APAC at BNP Paribas Securities.
"People feel more yen appreciation pressure at this moment, and that is obviously putting pressure on the Nikkei."
The dollar fell 0.2 percent to 109.41 yen, extending its 0.6 percent drop on Thursday.
"Earnings results season was okay, but geopolitical risk really cast a shadow over it, because the tension between the U.S. and North Korea is a never-ending story," which also bolstered the perceived safe-haven yen, Okazawa said.
While the fiery rhetoric between Washington and Pyongyang has calmed down, concerns remain that tension could again flare up on the Korean peninsula.
The United States and Japan will step up their defense cooperation to deal with the threat from nuclear-armed North Korea, officials from the two allies said on Thursday.
Banks and financial shares underperformed, with the banking subindex down 1.5 percent, the insurance subindex losing 2.1 percent, and the securities subindex 1.7 percent lower at midday.
Mizuho Financial Group and Mitsubishi UFJ Financial Group the first- and second-most traded issues by volume, fell 1.1 percent and 1.9 percent, respectively.
The broader Topix was down 1 percent at 1,598.23, poised to shed 1.2 percent for the week, while the JPX-Nikkei Index 400 was also 1 percent lower at 14,190.79, down 1.2 percent for the week.
Reporting by Tokyo markets team; Editing by Kim Coghill