MILWAUKEE — The Republicans gathered here are walking on air, as you would expect given the extraordinary events of the past month, which include their nominee, former president Donald Trump, surviving a close call with an assassin’s bullet.

Meanwhile, their opponents, the Democrats, continue to break the first law of holes: When you’re in one, stop digging. President Biden’s stumbling performance in the June 27 debate with Trump spectacularly reinforced the top voter concern about him — his age, 81, and fitness for another term.

Yet members of Team Biden have responded by calling in a backhoe. From the president on down, they insist he’s in it to win it. The numerous Democrats who say otherwise are being politely told to stick with the program, except when they’re being rudely told to “cut that crap out,” as Biden reportedly barked at Rep. Jason Crow of Colorado during a recent Zoom meeting.

The damage to Democratic prospects is not just a matter of the president’s repeated memory lapses and confusion; his weakened speaking voice; and his flashes of inappropriate anger such as the one Crow experienced.

Much worse is the harm to the Democrats’ brand as the party of facts, truth and science.

Republicans were supposed to be the party of Big Lies, the biggest being that Biden stole the 2020 election. Yet accepting Biden’s insistence on running requires Democrats to believe, or pretend to believe, a falsehood: that he’s sharp, fit and ready to govern another four years.

This is not to suggest moral equivalency between gaslighting about Biden’s age-related deficiencies and Trump’s lies about the 2020 election, which led to violence on Jan. 6, 2021.

Still, as whoppers go, “there is no reason to worry about Biden” is a good-size one, as confirmed by both basic medical knowledge regarding octogenarians and the evidence of ordinary people’s senses.

Above all, Biden’s continued candidacy implicitly discredits the main Democratic campaign theme: Democracy itself is on the line in 2024. Dire warnings about what will happen if Trump regains the White House can’t be both (a) valid and (b) consistent with knowingly running a flawed opponent against him.

There was plenty of time for Democrats to start organizing alternatives after their surprisingly good showing in the 2022 midterms. They neglected to do so, probably because of inertia, combined with an expectation that the GOP would not be able to bounce back from 2022, either under Trump or some other standard-bearer such as Florida Gov. Ron DeSantis. They assumed, wrongly, that criminal and civil cases would weaken Trump.

Now, a few family and staff enablers are reportedly reinforcing the president’s instincts, which are those of a career politician loath to surrender the ultimate prize.

Biden has taken to speaking of himself this way: “Name me a foreign leader who thinks I’m not the most effective leader in the world on foreign policy. Tell me! Tell me who the hell that is!” he reportedly said on that Zoom call with Crow and other moderate Democratic House members.

The loyalists might truly believe they are doing the right thing. It would be messy to replace Biden at this late date; a successor would not necessarily be more successful against Trump.

As optimists contend, Biden’s standing in national polls did not crater after June 27. He could win, despite everything, just as Democratic House and Senate candidates blew up the conventional wisdom about a “red wave” by winning in 2022.

But happy talk about Biden’s polls — some coming from Biden himself — overlooks the facts that major swings are rare in a deeply polarized electorate and that several post-debate polls showed downward ticks in his support.