Two months ago Trump reluctantly signed the Countering America's Adversaries Through Sanctions Act (CAATSA) despite concerns over the measure limiting his ability to lift sanctions without Congressional approval, as well as limiting his goal to improve relations with Moscow.
CAATSA passed both the US Senate and House of Representatives by near-unanimous margins large enough to override presidential veto power. But Trump's objections to the sanctions raised doubts as to whether his administration would enforce the measures.
The Senators stressed that the legislation required the administration to identify — by the start of October — those individuals operating on behalf of Russia's defense and intelligence sectors who could be subject to potential penalties, and urged Trump to meet that deadline.
"Now, as critical deadlines are approaching, it is imperative that your Administration implement the law to its fullest extent to uphold and protect American interests," the letter said, cited by Reuters.
"Congress' swift and united action, and your signature, sent a strong message to our allies and adversaries alike, and particularly to those such as Russia, who have sought to undermine our democracy," McCain and Cardin wrote, implying long-disproved allegations by many Democrats that Russia and Trump aides coordinated efforts to attack the credibility of Democratic candidate Hillary Clinton.
No evidence has surfaced of any role played by Moscow in the US 2016 presidential campaign that indicate Trump aides colluded with Russia.
McCain and Cardin stressed in their letter that Trump is obligated to demonstrate "from the highest levels that any effort to undermine US sanctions will not be tolerated" and warned that Congress would conduct "vigorous oversight" of CAATSA.
Later on Friday, the White House issued a presidential memorandum taking a first small step toward implementation, by designating different agencies to start the process that puts the law into effect.