Shares of former President Donald J. Trump’s social media company have fallen so much that his onetime $6 billion stake is now worth about $2 billion.

The stock price of Trump Media & Technology Group, at about $17.40 a share in early trading on Wednesday, is down more than 70 percent from the high-water mark it hit after Trump Media’s merger in March with a publicly traded shell company.

Mr. Trump is the single largest shareholder of Trump Media, the parent company of Truth Social, owning 115 million shares — a roughly 60 percent stake.

The slide in the share price has accelerated over the past few weeks as the presidential campaign has heated up and the Democratic nominee, Vice President Kamala Harris, has narrowly overtaken Mr. Trump in most national polls. Shares of Trump Media often have risen and fallen in tandem with investor perception of how Mr. Trump is doing in the presidential race.

The stock is also falling in advance of a pivotal date — the expiration of a contractual lockup that has precluded Mr. Trump from selling any of his shares. But on Sept. 19, he and other early investors can start selling shares, which could further depress Trump Media’s stock price.

Trump Media’s shares are now trading just around the $17.50 price that the shell company it merged with, Digital World Acquisition Corporation, had at the start of the year. In January, investors began buying shares of Digital World in anticipation of the merger’s completion.

By the time the merger with Trump Media closed on March 25, Digital World shares had risen to nearly $40, and they peaked two days later above $66. But the shares, even at their current level, still trade at a sky-high valuation given that Trump Media is losing tens of millions of dollars each quarter and Truth Social is taking in meager advertising revenues.

Many of Trump Media’s shareholders are individual investors and supporters of the former president who originally bought shares of Digital World after the two companies announced plans to a merger in October 2021. Some bought in when shares of Digital World were trading near $90 in the initial wave of exuberance surrounding the deal.