With his approval rating at a historic low, Donald Trump has brought down the curtain on the Republicans’ first ever midterm convention by pleading with supporters to do him “the biggest favour” and “pretend” they are voting for him in November’s US elections.

The president is facing a growing backlash over rising prices and the war in Iran. Sweeping losses for the Republican party could effectively turn Trump into a lame duck for his final two years in the White House.

Sensing political danger, Trump wrapped up the novel two-day event in Dallas, Texas, on Thursday with a pitch that critics are likely to say smacked of desperation: “You gotta do me the biggest favour you’ve ever done me. You’ve gotta do this for me. You’ve got to pretend I’m on the ticket. I’m on the ticket! I’m on the ticket! He’s on the ticket! Pretend I’m on the ticket because this is going to turn out to be one of the most important elections.”

In an improvised pledge of allegiance to himself, Trump directed the crowd of about 20,000 to raise their right hands and repeat: “I pledge to the greatest president in the history of the United States, that loves us so much he can’t even breathe, that I will go out with my family, my friends, I’ll do it any way – I don’t care if I’m registered or not, I’m going to try to cheat like hell like they do.”

The convention had been aimed at turning around Republicans’ fortunes. Opinion polls suggest they are on course to lose the US House of Representatives, and possibly the Senate.

In what was effectively an abridged version of his rambling keynote address on Wednesday, the president reiterated his promise of “Trump dividend” of $5,000 for every US adult if Republicans win both chambers of Congress. Democrats have dismissed the pledge as a bribe while economists have warned it would cost more than $1tn.

Trump also hammered the theme of the convention: drawing a contrast between his own accomplishments and what he argues amounts to a threat posed by Democrats.

“Less than two years ago, the American people gave us a mandate to save America – and in 19 months, I’ve delivered on every single promise I made,” he claimed. “They gave us wide open borders with 25 million people pouring in from all over the globe. They turned our cities into scenes of death and misery, murder, poverty and destruction, and made America a laughingstock all over world.

“But we took that dark age of America and turned it into the golden age of America.”

The reduction of prescription drug prices alone “should get us a big victory in our midterms”, he suggested, before he turned the microphone over to the opera performer Christopher Macchio, who belted out a soaring rendition of America the Beautiful as red, white, blue and golden balloons cascaded down from the ceiling of the basketball arena.

The convention resembled a Trump mega-rally more than a traditional party meeting, with no nominations or formal business and more than 50 speakers or presentations each night. Supporters wore “Make America Great Again” regalia and waved signs displaying the Stars and Stripes. Republicans believe it has been successful in galvanising the party’s base in the face of stiff electoral headwinds.

Earlier on Thursday, speaker after speaker sought to champion Trump’s accomplishments while constantly branding the Democratic party as extremist and communist.

The president’s eldest son, Donald Trump Jr, delivered a fiery address setting Republican “common sense” against a “dystopian hellscape” of leftwing ideology.

“As we speak, Democrat candidates across the country are openly embracing the radical ideology of democratic socialism,” suggested Trump Jr. “They claim that their socialism is for the people, but that’s the exact same story we’ve heard from every communist in human history.”

Describing childhood memories of his grandparents in communist Czechoslovakia, he warned of bread lines and rationing. “Communism is on the march and it’s up to us to stop it in its tracks,” he said.

Trump Jr framed the Republican platform, including pledges to end taxes on tips, overtime and social security, as a direct response to a “war on the working people” by Democrats. “The choice is clear,” he said. “Don’t let these radicals gaslight you into thinking common sense is extreme, and extreme is somehow common sense. Don’t let them convince you that caring about this country makes you a villain.”

Senator Ted Cruz meanwhile warned of a “red-green alliance” between “communists and Islamists” who “hate Christians, hate Jews and hate capitalism”. Invoking his father’s escape from Cuban communism, Cruz declared that “sharia law has no place in America” and was “fundamentally inconsistent with the constitution”.

The House speaker, Mike Johnson, echoed the conspiratorial tone, suggesting the nation was being “attacked from within”. The Democratic party is now beholden to radicals, he claimed, warning: “They’re the ones who have let the barbarians inside the gate, and next year the communists will be in Congress. That’s what’s so frightening.”

Johnson added that, while big names like Zohran Mamdani and Abdul El-Sayed are known entities, the bigger threat comes from “Democrats in the swing seats who say one thing, and then join forces and vote with the socialists that they’re afraid of”.

The US health secretary, Robert F Kennedy Jr, took the stage to champion the “Make America Healthy Again” (Maha) platform. The son of a former US attorney general and nephew of John F Kennedy said the “irony of my presence here is inescapable”.

He continued: “The Democrats of that era stood with labour and working people and the American middle class. Nearly every cop and firefighter was a Democrat … The Democratic party, in short, lost its way.”

Speaking less than a mile from the spot where his uncle was assassinated in 1963, Kennedy claimed it was a “fact” that “neither my father, nor my uncle, would recognise the Democratic party of today”.

JD Vance, the vice-president, gave the keynote address of the night. He said: “The Democrats are the party of hatred. They’re the party that despises America and the people who built it … America belongs to those who love it – not those who hate it.”

The political attacks were briefly paused for two sombre interludes.

A tribute marked one year since the assassination of the rightwing provocateur and organiser Charlie Kirk. Lucas Miles, a Turning Point USA vice-president, said that while “they could take the man, they could not take the mission”. Chants of “Charlie, Charlie” filled the air.

The arena was plunged into darkness as Macchio performed Ave Maria on the eve of the 25th anniversary of the September 11 terrorist attacks. Two beams of light stretched upward from the arena floor, reminiscent of the twin towers in New York, as the crowd held up mobile phone lights. A video narration accompanying the tribute urged the audience to: “Say their names. Honour their sacrifice. Defend Freedom. Never forget September 11.”

Democrats have dismissed the Republican event as a stunt. At a press conference in Dallas, Robert Garcia, a California congressman, told reporters: “What’s happening right now is Donald Trump is throwing a two-day party to celebrate himself, so that JD Vance can give a speech, and so his donors, some of whom have paid $800,000 a pop for these ticket packages, can spend two days celebrating themselves.

“This is happening as the American public is struggling. So as Donald Trump and JD Vance party away for two days, the average American family can’t afford groceries, the cost of the rent’s going up, they’re looking at their gas prices skyrocketing and they’re making difficult choices at the dinner table. But Donald Trump does not care about the average American family or about affordability.”