One policy issue will dominate this presidential election like no other: policing the border with Mexico.

• Both President Biden and former President Trump are pouncing on the topic in the unofficial start to the 2024 general election.

Why it matters: Their internal polls show what public ones shout — that immigration is the policy topic animating voters most viscerally.

The issue has made both candidates say and do extraordinary things in the past few days.

• Trump opposes a bipartisan border security bill that would give Biden emergency border-security funding.
• "I'll fight it all the way," Trump said Saturday in Las Vegas, in what aides considered his first speech of the general election. "Please blame it on me. Please."
• Biden is all-in on border restrictions he would have ridiculed a few short years ago, saying Saturday in Columbia, S.C., that the bill would give him "the emergency authority to shut down the border until it could get back under control. If that bill were the law today, I'd shut down the border right now and fix it quickly."

Truth bomb: Republicans know immigration alone could sink Biden. So Trump and House Republicans will kill anything, even if it meets or exceeds their wishes.

• Biden knows immigration alone could sink him. So he's willing to accept what he once considered unacceptable — to save himself.

Go deeper: Biden on the trail ... Trump on the trail.