"Your memorandum explicitly covers new and expanded pipeline projects so we were confused and disappointed to learn that the Keystone XL pipeline would not be required to use 100 per cent American-made steel," the nine Senators wrote in their joint letter to Mr Trump, a copy of which was released to the press yesterday.
"Further, we are deeply concerned that by allowing this Canadian firm to use foreign steel from countries like India and Italy, which have a history of dumping steel products in the US market at unfair, illegal prices, you are establishing a precedent that will have the effect of costing US jobs and undermining the spirit of your Presidential Memorandum," the Senators wrote.
Led by Senators Chris Van Hollen and Tammy Duckworth, the Democratic lawmakers urged Mr Trump to protect American jobs by ensuring all new pipelines - if approved - are constructed and maintained with American made products and equipment.
Other signatories to the letter are Cory A Booker, Thomas R Carper, Al Franken, Christopher S Murphy, Debbie Stabenow, Joe Donnelly Claire McCaskill, Robert Menendez, and Gary C Peters.
"As champions of expanding Buy American requirements to make sure taxpayer-supported projects contract with American companies to the greatest extent possible, we were initially encouraged by this memorandum," they said.
"We were disappointed, however, when we learned that your administration would exempt the Keystone XL pipeline project from this Buy American policy," the letter added.
On January 24, Mr Trump issued a Presidential Memorandum to the Secretary of Commerce Wilbur Ross directing him to "develop a plan under which all new pipelines, as well as retrofitted, repaired, or expanded pipelines, inside the borders of the US, including portions of pipelines, use materials and equipment produced in the US, to the maximum extent possible and to the extent permitted by law."
However, Mr Trump has exempted Keystone pipeline from this.
"We request that you reconsider your decision to allow a foreign company to use foreign steel in the Keystone XL pipeline and urge you to secure a firm commitment to source 100 per cent American-made steel for this project," the Senators wrote.