Anti-trans ads targeting Kamala Harris are flooding the airwaves in the closing stretch of the election, as Donald Trump seeks to win over undecided voters by attacking an already vulnerable community.

The Trump campaign and Republican groups have poured tens of millions of dollars into the inflammatory TV ads, which have aired in key battleground states and during nationally-broadcast NFL games that draw a strong viewership.

The advertising blitz suggests Republicans are banking on "culture war" messaging to move the needle in a US election that is still too close to call.

"Kamala supports taxpayer-funded sex changes for prisoners," a female narrator says in one of the ads.

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"Kamala is for they/them. President Trump is for you," she adds, referring to the pronouns used by some transgender and non-binary people.

The ad ends with Trump's voice, asserting that he "approves this message."

"What is most alarming is the size and scope of these ad campaigns – comprising some of the GOP's largest TV ad investment," Imara Jones, chief executive of the nonprofit TransLash Media, told AFP.

"These ads, mostly focused on the healthcare needs of trans inmates, are designed to trigger deep fear" among voters, added Jones, who is herself a Black trans woman.

Over the first half of October, the Trump campaign and its allies spent $21 million on ads attacking Kamala Harris over "LGBTQ rights," CNN reported, citing data from the media tracking agency AdImpact.

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That is nearly one-third of their total spending on broadcast TV ads in that period, AdImpact said.

Nearly all the ads featured clips of Harris from four years ago expressing her support for gender-affirming care for federal prisoners and detained immigrants.

Lost in the discourse is former president Trump's own record. Officials under his administration also offered some inmates an array of gender-affirming treatments.

Earlier this month, a Gallup survey of registered voters found that 38% of Americans said a candidate's position on transgender rights was "extremely" or "very" important to them. But it ranked last among about two dozen leading topics that resonate with voters such as the economy, immigration, education, health care, and abortion.

That chimed with another recent study by the advocacy group GLAAD and Ground Media that the anti-trans ads campaign triggered "no statistically significant shift in voter choice, mobilization or likelihood to vote."

"What this demonstrates is that attacking the trans community isn't just a weak and feckless political strategy – it's a deeply cynical one," said David Rochkind, chief executive of Ground Media.

"These ads weaponize trans-identity to sow fear and division, making our country less safe for everyone."

The study warned that the ads could have potentially "harmful consequences" for trans Americans, with its participants reported feeling less accepting towards the community after being exposed to the campaign.

In recent years, the transgender community has been a growing target among conservatives, with Republican lawmakers introducing bills across the country to limit gender-affirming care, bathroom access, and their ability to participate in sports.

The anti-trans ads are "designed to rile up the Republican base," Todd Belt, director of the political management program at George Washington University, told AFP.

"It has very limited appeal to undecided voters, and often comes off as mean-spirited," Belt said, adding that many Americans were tired of the "culture war playbook."

The issue, however, does resonate with Trump's core base, many of whom are vehemently opposed to transgender athletes competing in women's sports.

Drawing cheers and applause at his rallies, Trump has pledged to fight "transgender insanity" and to "keep men out of women's sports."

More than half of all Americans believe changing genders is "morally wrong," according to another Gallup survey.

"In an election where every vote counts, Republicans are betting that these ads will move the needle with a small set of voters in tight races where a few votes make a big difference," TransLash Media's Jones said. "They know that these messages are effective at moving voters on the margins."