Senator Cardin called on external actors to stop arming the belligerents in Sudan and called for a diplomatic solution to the conflict there.
President Mnangagwa, 81, is serving his second and last term after succeeding long time ruler Robert Mugabe following a military coup in 2017.
South Africa will hold the national and local government elections on May 29.
There has been uncertainty around Russia's allegiances in Sudan due to its ties with RSF Commander Mohamed Hamdan Daglo.
UN sanctions monitors have described as "credible" accusations that the United Arab Emirates had provided military support to the RSF.