"These are all major accomplishments that will begin to move the country in the right direction," Sanders writes.
But he goes on to say the document, which the full drafting committee will vote on again before the party's convention this month, needs to be "significantly improved."
He says that one of the most important amendments that his delegates on the committee will offer is "to make it clear that the Democratic Party is strongly opposed to the Trans-Pacific Partnership."
Sanders suggests in the op-ed that his members on the committee had offered an amendment to state the party's opposition to having the TPP voted on during the lame-duck session of Congress, but that the amendment was defeated with all of Clinton's delegates voting against it.
"I don't understand that because Clinton, during the campaign, made it very clear that she did not want to see the TPP appear on the floor during the lame-duck session," Sanders writes.
In addition to the op-ed, the Sanders campaign website has launched a petition asking signers to "Tell the DNC Platform Committee: Strongly Oppose the TPP in the Party Platform."
The Clinton campaign responded to ABC News' request for comment by forwarding a recent statement the candidate made on TPP in response to criticisms from the Trump campaign: "Hillary Clinton opposes TPP today, she will oppose it in November, and she would not move it forward in January. Hillary Clinton has made clear that she is not interested in tinkering around the margins with TPP. As she has said repeatedly, she believes we need a new approach to trade that protects American jobs, raises incomes for American workers, and strengthens our national security."
A surrogate for Clinton, Ohio Democratic Sen. Sherrod Brown, also on Sunday addressed the candidate's stance on TPP.
Brown said on ABC's "This Week" that Clinton previously supported TPP because as secretary of state for President Obama she was advocating a trade policy he supported.
"It was her boss," Brown said. "She was the secretary of state for the president. Of course she's going to the take those positions."
"I trust her on this," he added.
The platform-drafting committee on June 24 voted down an amendment opposing the TPP that had been proposed by Rep. Keith Ellison of Minnesota, a Sanders delegate on the panel.
Other members of the committee said it would be wrong for the platform to undercut the outgoing Democratic president, who has promoted the TPP.
"What I don't want to do is leave this place disregarding the position of the president of the United States," said Illinois Rep. Luis Gutierrez, a Clinton supporter.