The FBI’s raid on Mar-a-Lago on Monday has galvanized the American right, raising the prospect that Donald Trump’s grip on the Republican party could strengthen at a time when the former president had been losing it.

After Trump announced in a statement that his resort and residence was “currently under siege, raided, and occupied”, angry supporters rushed there to protest as police with rifles looked on. “All the media are against Trump, and I’m fed up with it,” a supporter holding a sign saying “Fake news” told a Reuters reporter.

Trump’s daughter-in-law Lara Trump declared on Fox News that the raid – which sources previously told the Guardian was in search of documents that the ex-president may have taken from the White House – was intended to foil him from running for the Oval Office again.

The political establishment is “terrified he’s going to announce any day that he’s running for president in 2024”, she said, “and this is a very convenient way to just throw a little more mud on Donald Trump, as though they haven’t already done enough”.

Trump’s influence on the Republican party had appeared to wane somewhat in recent months. His interventions in the Republican primaries have had mixed results. Some of his endorsed candidates won – often in tight races – and others lost.

Similarly, Republican voters’ enthusiasm for the idea of Trump running for president again had been declining. A New York Times/Siena College poll in July found that almost half of Republican primary voters preferred someone other than Trump for 2024. Younger Republicans and those with college degrees preferred Florida’s governor, Ron DeSantis, who has worked in recent years to position himself as Trumpism’s shrewder and more disciplined heir.

Now the US justice department may have handed Trump a gift in disguise. Trump could download footage of the raid from his home security cameras and “have one of the most gripping campaign ads of all time, ready made”, the novelist Walter Kirn suggested on Twitter.

By Tuesday morning, Trump had already posted a lengthy campaign-style ad to social media describing the US as “a nation in decline” and saying that the country “has weaponized its law enforcement against the opposing political party like never before”.

His “Save America” Pac blasted out fundraising texts directing supporters to a landing page reading: “It’s time for EVERY PATRIOT to step up and stand against the Left’s reckless WITCH HUNTS and political persecution of President Trump! Please rush in a donation IMMEDIATELY to publicly stand with President Trump against this NEVERENDING WITCH HUNT!”

A person close to the Trump operation told Politico: “They’re going to drastically use this to rally their allies, [Republican] leaders on Capitol Hill and juice for his political agenda and run for 2024.

“If there was a 99% chance” of Trump running again, “it’s 100% now,” Politico’s source added. The outlet also reported that the Trump camp was keeping a careful eye on which Republicans rally to the former president and which do not.

The Republican National Committee has seized on the news as ammunition for midterm elections this fall, with fundraising texts reading: “THIS IS NOT A DRILL: UNPRECEDENTED move [Joe] Biden’s FBI RAIDS Pres. Trump’s home. Time to take back Congress.”

DeSantis, who is widely viewed as a major obstacle to Trump if he were to run again, came to his rival’s defense, though without mentioning Trump directly by name. The raid on Mar-a-Lago “is another escalation in the weaponization of federal agencies against the Regime’s political opponents, while people like Hunter Biden get treated with kid gloves”, he said on Twitter, adding that America was becoming a “Banana Republic”.

The Republican Florida senator Marco Rubio disparaged the raid on similar lines. “Using government power to persecute political opponents is something we have seen many times from 3rd world Marxist dictatorships,” he said on Twitter. “But never before in America.”

In Townhall, a rightwing online magazine, a columnist argued: “If Trump was ever on the edge of not running, and I don’t think he ever was, then this act sure as hell is going to make him run again in 2024.

“The FBI, not the media, may have given the most significant in-kind political contribution to a candidate in American political history.”

A column in RedState, another conservative website, warned fellow Trump supporters: “Don’t Take the Bait.” The column sought to portray the raid as a last-ditch effort to boost Democratic prospects for this fall’s midterm elections.

“Why?” asked the column. “Because they believe it can save them from certain destruction in November.”