The billionaire owner of the social media platform X reposted a video that mimics Vice President Kamala Harris’s voice, without disclosing that it had been altered.

Elon Musk, the world’s richest man, has waded into one of the thorniest issues facing U.S. politics: deepfake videos.

On Friday night, Mr. Musk, the billionaire owner of the social media platform X, reposted an edited campaign video for Vice President Kamala Harris that appears to have been digitally manipulated to change the spot’s voice-over in a deceptive manner.

The video mimics Ms. Harris’s voice, but instead of using her words from the original ad, it has the vice president saying that President Biden is senile, that she does not “know the first thing about running the country” and that, as a woman and a person of color, she is the “ultimate diversity hire.”

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Pro-democracy groups have raised increasingly urgent alarms about deepfakes, a broad term for digital content that employs artificial intelligence and other technology to create audio, video or images that spread false information and could influence voter behavior.