Short of an election-eve exoneration by Robert Mueller, it would be hard to imagine a nicer October surprise for Donald Trump than an attempt by thousands of unauthorized immigrants to force the borders of the United States. It dramatizes every one of his themes, but none more spectacularly than this: his claim that his opponents will not defend the borders of the United States.

On Sunday, some thousands of people rafted across the Suchiate River, which separates Guatemala from Mexico. Mexico did not detain or expel them, and soon they were on the move again. Organizers seem to hope that the unprecedented mass of the caravan will overawe Mexican and U.S. authorities.

What it is also doing is testing the U.S. political system.

Just as Donald Trump has energized American conservatives to take a harder line on immigration, he’s had the opposite effect on his political opponents. It is, of course, crazy for the president to describe the caravan as “led” by Democrats, as he did in a tweet last week. But he’s not wrong to sense that Democrats are massively discomfited by the caravan. Trump’s tweeting and fulminating and conspiracizing deters any Democratic leader from saying or doing anything that could be interpreted as agreeing with him.