Former President Donald J. Trump laid out what’s in store for America should he or President Biden win the 2024 presidential election, using a Saturday speech at the Conservative Political Action Conference to cast one nearly utopian vision of the country’s future and one reminiscent of a postapocalyptic movie.

If Mr. Biden is re-elected for a second four-year term, Mr. Trump warned in his speech, Medicare will “collapse.” Social Security will “collapse.” Health care in general will “collapse.” So, too, will public education. Millions of manufacturing jobs will be “choked off into extinction.” The U.S. economy will be “starved of energy” and there will be “constant blackouts.” The Islamist militant group Hamas will “terrorize our streets.” There will be a third world war and America will lose it. America itself will face “obliteration.”

On the other hand, Mr. Trump promised on Saturday that if he is elected America will be “richer and safer and stronger and prouder and more beautiful than ever before.” Crime in major cities? A thing of the past.

“Chicago could be solved in one day,” Mr. Trump said. “New York could be solved in a half a day there.”

It’s impossible to fact-check the future. But Mr. Trump’s speech at the Gaylord National Resort and Convention Center in Maryland sounded familiar — like 2016 or 2020 all over again.

In his 2020 campaign, Mr. Trump warned that Mr. Biden would “confiscate your guns,” and “destroy your suburbs.” He predicted that the economy would sink into a depression worse than the 1930s Great Depression and that the “stock market will crash.” A Biden presidency, he predicted four years ago, “would mean that America’s seniors have no air conditioning during the summer, no heat during the winter and no electricity during peak hours.” And, he warned in July 2020, “you will have no more energy coming out of the great state of Texas, out of New Mexico, out of anywhere.”

Some of those past predictions are now checkable, and have turned out to be fictions. The stock market has hit record highs under the Biden administration. Guns haven’t been confiscated. Air conditioning is as good or bad as it ever was. And under Mr. Biden, the United States is producing more oil — not only more than it did under Mr. Trump but more than any country ever has.

Mr. Trump also left office with a long list of his own unfulfilled campaign promises, including completing the construction of a wall along the southwestern border. On Saturday, he pinned the blame for that failure on fellow Republicans in Congress — and on his own inexperience.

“Don’t forget, I had never done this stuff before,” he said, describing his border wall negotiations.

Still, Mr. Trump’s vision of the country delivered at CPAC on Saturday has the potential to connect powerfully to the fears and lives of millions of Americans.

When Mr. Trump said on Saturday that Mr. Biden had allowed “hordes of illegal aliens stampeding across our borders,” he was speaking to a voting public that trusts Mr. Trump significantly more to handle immigration. Under Mr. Biden, record numbers of undocumented migrants have crossed the southern border, straining local services and infuriating even Democratic mayors and governors, who have pleaded with the White House to take the problem more seriously. (Mr. Trump did not mention in his speech how he has all but killed a bipartisan effort to help solve the problem because he wanted to deprive Mr. Biden of a legislative victory in an election year.)

And when Mr. Trump rails against what he portrays as a bad economy under Mr. Biden, his message empirically resonates with voters even if the Biden administration can point to any number of economic data points to brag about. Under Mr. Biden, unemployment is low, real wages are rising, the stock market is booming and inflation is finally cooling. But at the same time, many groceries and other living expenses are vastly higher now than they were under Mr. Trump. When Mr. Trump hammers Mr. Biden for inflation, as he often does, he taps into an issue that Democratic strategists fear as one of Mr. Biden’s biggest liabilities this fall.

On Saturday, after delivering a series of dire warnings about a second Biden term, Mr. Trump ditched his prepared remarks to share long, rambling anecdotes about what he portrayed as his brilliant behind-the-scenes negotiating as president. “Nobody can ramble like this,” he said of his own rambling, as he brought up his late uncle.

For his part, Mr. Biden has delivered his own warning, telling supporters that Mr. Trump would undo America’s democratic principles and be an agent of chaos if he returns to the White House. Last month, on the eve of the third anniversary of the Jan. 6, 2021, attack on the Capitol by Mr. Trump’s supporters, Mr. Biden said in a speech, “There’s no confusion about who Trump is or what he intends to do,” adding, “We all know who Donald Trump is. The question is: Who are we?”

Mr. Trump’s CPAC speech came on the day of the G.O.P. primary election in South Carolina, the home state of his main Republican rival, Nikki Haley. He has dominated the primary race so much, and was leading Ms. Haley in polling averages by so many points, that Mr. Trump adopted the rhetoric and posture of a front-runner ignoring the primary and focusing on the general election in November. Not once in his entire speech did he say Ms. Haley’s name.

What made Saturday’s speech different for Mr. Trump from the 2016 and 2020 versions was how he has turned his unprecedented legal situation, as the first former president charged with crimes, into a core part of his campaign message. Even as Mr. Trump now insists that his only “revenge” will be success for the American people — a departure from his previous promises to direct the prosecutions of his political opponents — the theme of retribution coursed through CPAC.

“I stand before you today not only as your past and future president, but as a proud political dissident,” Mr. Trump said.

“For hard-working Americans,” he added, “Nov. 5 will be our new liberation day — but for the liars and cheaters and fraudsters and censors and impostors who have commandeered our government, it will be their judgment day.”

At that, the crowd whistled and roared.