With talks to begin in August to renegotiate the North American Free Trade Agreement, dozens of industry, agriculture, labor and economic officials offered their views to the Trump administration on Tuesday. 

The US Trade Representative opened three days of public hearings that were sure to feature many conflicting views on the 23-year-old trade pact.

Industry representatives with a stake in cross-border trade, such as farming and manufacturing, were due to be joined by critics, including labor rights groups.

President Donald Trump has described NAFTA as a "disaster," but backed away from his threat to withdraw the United States from the trade deal with Canada and Mexico after discussions with his counterparts.

The administration now is focused on modernizing NAFTA and getting the best possible deal for US companies, and is seeking input on how to achieve that.

Prior to the hearing, the USTR website was deluged with more than 12,000 comments on the renegotiation which has reanimated long-standing political arguments on trade liberalization. 

And just before the testimony began, an anti-trade coalition -- which claims to have crashed the USTR comment website -- delivered stacks of petitions calling for a better deal for US workers and demanding Trump keep his campaign promise to renegotiate the pact.

"We, along with the hundreds of thousands of Americans who have signed these petitions, will continue to demand a new, comprehensive approach to trade policy under the Trump administration," said Democratic congresswoman Rosa DeLauro said in a statement.

In his testimony, Congressman Sandy Levin, a Democrat from Michigan, told the hearing that US autoworkers' standard of living had suffered because of unfair competition from Mexico.

"The entire Mexican labor system is designed to prevent workers from obtaining their rights and bargaining for better wages and working conditions," he said in his prepared remarks.

However, Chip Councell, chairman of the US Grains Council and a Maryland farmer, said the trade pact is important to many like him.

"The last several months have highlighted how important it is to maintain this strong, stable relationship if we are going to continue to grow," Councell said in his prepared testimony.

He said Mexico's trading partners were concerned for the future of NAFTA and those jitters were depressing sales of US corn, sorghum and barley.

"Strong but unconfirmed evidence" suggested Mexican farmers already were preparing to buy corn from South America later in 2017 instead of from the United States, he added.