US President Donald Trump is reviving Cold War-style warnings about communism, casting the Democratic Party as a red "menace" as Republicans hunt for a midterm message to hold their narrow grip on Congress.

The rhetoric, echoing the anti-communist crusades of the 1950s, has surged from Trump's speeches into the broader Republican campaign machine after a string of democratic socialist primary victories in New York and Colorado.

And it has grown apocalyptic, often overtaking the president's fixation on election fraud, as he frames November's vote as a civilizational clash between Republican "common sense" and left-wing extremism.

During weekend speeches at Mount Rushmore and in Washington marking America's 250th anniversary, Trump warned of a communist "menace" that needed to be cut out "like a cancer."

At a recent religious conference he went further, accusing the Democratic left of being "hardcore, godless communists" who wanted to "completely destroy the traditional American way of life."

The rhetoric reflects a sharpening midterm strategy for Republicans facing frustration over inflation, affordability and the fallout from Trump's war with Iran. But it conflates democratic socialism -- which operates within elections and a market economy -- with communism, associated with central planning, one-party rule and abolishing private ownership of major industries.

Analysts say the party is trying to turn the elections from a referendum on Trump into a choice between two ideologies, using the rise of the far-left to paint all Democrats as radical.

"Trump's (Republican Party) is perceived by many voters -- particularly independents -- as too extreme in their policies," Daniel Drezner, a politics professor at Tufts University in Massachusetts, told AFP.

"One way to address that before the midterms is to paint the opposition party as even more extreme."

The Washington Post reported that Trump's allies have sharply increased their online use of "communist" and "communism," with average weekly mentions up 43 percent from a year ago.

Senior Republicans have echoed the message. House Speaker Mike Johnson warned Sunday that "barbarians are inside the gate" while Trump spokeswoman Karoline Leavitt called the election a choice "between communism and common sense."

The push follows a handful of far-left primary wins that have unsettled Democrats and given Republicans a potent attack line in the fight for Congress.

- McCarthyist speech -

Rick Stengel, an official in the administration of Barack Obama, joked that Trump's Independence Day remarks sounded as if the White House had just "discovered a July Fourth Joseph McCarthy speech from 1952."

Senator McCarthy led a 1950s crusade against alleged Soviet infiltration, ruining careers and turning "communist" into one of the most explosive accusations in US politics.

His chief counsel, Roy Cohn, later became Trump's mentor.

Republicans argue the left's victories have given them a vivid contrast for swing districts, especially with Trump's approval weak, his signature tax and spending law deeply unpopular and the Iran war fueling price anxiety.

Some left-leaning analysts also say "wokeness" -- framed by its opponents as a hectoring form of political correctness -- has made Trump's attacks easier.

Spencer Critchley, a communications consultant who worked on both of Obama's presidential campaigns, says the ideology has attracted actual communists and coincides with young Americans' growing, if muddled, support for socialism.

"So while in no sense is the Democratic Party communist, it's become easier for Trump and his enablers to spread the lie that it is," Critchley told AFP.

Others argue however that "democratic socialist" no longer scares voters as it once did, particularly younger Americans who came of age after the Cold War.

Brad Chase, a veteran communications and crisis management strategist, sees Trump's communism fixation as a messaging misstep.

"Communism is a word that just doesn't resonate anymore" for Americans under 50, Chase told AFP.

Democrats in any case say the attack is a distraction from pocketbook concerns and Trump's own vulnerabilities.

"The reason Trump reached all the way back to Karl Marx at Mount Rushmore is that he's got nothing to say to a 28-year-old who can't make rent," progressive commentator Thom Hartmann told AFP.

"And the billionaires bankrolling his party would much rather talk about Marx than about who actually took that young person's future."