We can’t overstate the severity of President Trump’s buyer's remorse from signing last month's spending bill. It could even be a turning point in his presidency, on the issue of immigration and his level of cooperation with Republican leaders; Sources who’ve discussed it with Trump say it freaked him out to see the array of usually friendly faces on Fox News’ opinion shows ripping into him for signing a bill that spent a ton of money, but gave lots away to liberal priorities and did little for his signature promise to build a wall.

Why this matters: Truth is that Trump had little clue what was in the largest spending bill ever passed. Conventional wisdom on Capitol Hill has been that nothing will happen on immigration after the early failure to cut a deal this year. Republican leadership sources were telling us that the court decision to keep DACA alive took away Trump’s deadline and removed the pressure on Congress to act. But now some of those sources are nervous, realizing that Trump won’t let the issue fade into the background.

Trump wants action to toughen immigration laws, and he’s hopping mad that it hasn't happened. He’s grasping at whatever executive tools are available. But watch for him to force action before the end of September, when the government funding expires.

• A lot could change between now and then. But based on what Trump has been telling confidants, we expect a showdown — and possibly a shutdown — then. 

Be smart: An election eve fracas that forces Republicans to contemplate a government shutdown vs. harsher immigration laws will motivate some Republican voters. But it could further endanger vulnerable Republican incumbents in swing seats, in a campaign year already stacked against them.