I understand "Trump's uncontrolled rumbling" as "Trump's uncontrolled pompous behaviors". Am I on the right track?
But is there a dictionary that defines "rumbling" as such?
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Walter Hinteler wrote:
Trump's uncontrolled rumbling could be a clear signal that he has virtually abandoned his interim tactic of addressing floating voters in the suburbs. Now, apparently, his only concern is to get as many of his white working-class supporters as possible to the ballot boxes in important states like Pennsylvania, Florida or Wisconsin, to make up for probable losses in the country's better-off suburbs.
It is the tactic of a desperate man.
Source: monitoring Trump and relevant contemporary events