President Donald Trump accused China on Wednesday of pulling back from agreed-upon trade deal language because Beijing would rather negotiate with former Vice President Joe Biden "or one of the very weak Democrats" hoping to win the White House.

Trump launched his attack in a pair of tweets that moved equities markets.

"The reason for the China pullback & attempted renegotiation of the Trade Deal is the sincere HOPE that they will be able to 'negotiate' with Joe Biden or one of the very weak Democrats, and thereby continue to ripoff the United States (($500 Billion a year)) for years to come," Trump tweeted. "Guess what, that's not going to happen!"

The tweets showed that Trump's attention is on Biden, the Democratic front-runner, even as he prepares for the most crucial trade talks of his presidency. Since Biden joined the race last month, Trump has attacked him more forcefully than any other Democrat running for president.

Shortly after the tweets were posted, Biden's campaign responded.

U.S. trade policy, and in particular Biden's record as a vocal advocate of free trade and multilateral agreements, has already surfaced as a top issue in the both the Democratic primary and increasingly in Trump's still-evolving 2020 campaign platform.

In 2016, Trump's tough rhetoric on trade, and his pledge to renegotiate what he often called "disastrous" free trade deals like NAFTA, were critical to his strategy of winning over disaffected Rust Belt Democrats by promising to bring manufacturing jobs back to the United States.

With the 2020 election just 18 months away, Trump is under ever-increasing pressure to deliver on those promises.

By painting his Democratic rivals as too "weak" to successfully negotiate with the world's second largest economy, Trump appears to be trying to shift the narrative away from the idea that he and his White House are competing against the Chinese, and toward the idea that the real contest is between Biden and himself.

As his third major presidential campaign gets underway, Biden is already drawing criticism from the left of his party for playing down the threat Chinese competition poses to U.S. union-backed manufacturing industries. At a campaign stop in Iowa last week, he appeared to dismiss concerns about China's geopolitical ascendancy. "China is going to eat our lunch? Come on, man," he said.

Biden's rivals, meanwhile, have a veritable treasure chest of Senate votes Biden cast over three decades, as well as Obama administration trade policies, with which to draw contrasts between themselves and the former vice president.

"When people take a look at my record versus Vice President Biden's record, I helped lead the fight against NAFTA; he voted for NAFTA," Vermont Sen. Bernie Sanders told CNN shortly after Biden launched his campaign. "I helped lead the fight against [permanent normalized trade relations] with China; he voted for it. I strongly opposed the Trans-Pacific Partnership; he supported it."

Sanders also slammed Biden's more recent comments on China. "It's wrong to pretend that China isn't one of our major economic competitors," the senator said.

At other times, however, Biden has borrowed a line that Trump frequently used in his 2016 campaign, describing himself not as a "free trader," but as a "fair trader."

"I'm a fair trader," Biden said last week in Iowa. "That's why I've been arguing for a long time that we should treat other countries the way in which they treat us, which is, particularly as it relates to China: If they want to trade here, they're going to be under the same rules."

— CNBC's Jacob Pramuk contributed to this report.