回答
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Mr. Trump hasn't stopped making reckless remarks, even after he won the Republican Party nomination.
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Trump hasn't been able to control his mouth, even after becoming the GOP nominee.
「GOP」= Grand Old Party (アメリカの共和党のこと)
候補に選ばれた= to be nominated, to become the nominee
to control (one's) mouth = 悪い事を言わないということ