Does President Trump care if Republicans lose the House of Representatives this November? If that seems like an odd question, consider that Mr. Trump is running a campaign strategy that puts the House at maximum risk while focusing on the Senate. The latest evidence is Mr. Trump’s threat to shut down the government in September if he doesn’t get money for his border wall.

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It’s always risky to use the word “strategy” about Mr. Trump because he’s so impulsive and capricious. Only last week GOP leaders thought they had his...