U.S. Treasury Secretary Steven Mnuchin signaled Friday that the administration’s goal of completing a tax overhaul by the end of 2017 may not be a firm deadline.

“I think you know earlier in the year I said I’d thought we would get it done by August and I was wrong,” Mnuchin said during a press conference at the White House. “I’m now going to say that I’m very hopeful and I think we can get this done by the end of the year, but we will continue to revisit that.”

House Speaker Paul Ryan has said it’s essential to complete a tax bill by the end of the year or it risks being derailed by political complications surrounding mid-term elections in 2018. Senate Majority Leader Mitch McConnell hasn’t committed to finishing legislation by the end of 2017; in May, he declined to put a deadline on it.

The so-called Big Six -- made up of White House officials and congressional leaders involved in tax negotiations -- jointly released a two-page statement in July that outlined a broad set of agreed-upon tax principles. Specifics, including such basic matters as where to set the corporate tax rate and how to set up individual tax brackets, have yet to emerge.

Mnuchin’s remarks came as President Donald Trump is preparing to spend the next several weeks leading a public campaign in support of tax legislation. The White House will leave Republican lawmakers to hash out details of the plan, National Economic Council Director Gary Cohn said in an interview with the Financial Times.

Trump will kick off the campaign next week with a visit to Missouri, according to an administration official familiar with the plans. His trip Wednesday to the state’s southwestern city of Springfield is expected to be the first of several stops around the country in the coming weeks, said the official, who asked not to be identified because the details were still under review.

Cohn said in that interview that he believes the legislation can be written and pass both chambers of Congress by the end of the year.

“They have been holding hearings for years,” he said. “It’s not like they are just starting the process now.”