So how will 'the Trump administration' be "blocking a large number of voters who oppose the president from casting ballots" when all the voting rules, with the exception of Supreme Court rulings, are set by individual States?  

As the US Constitution permits.  

Oregon has chosen 'vote by mail'.  Other States have all kinds of voting methods.  Trump can't change Oregon's system.  Only our voters can.  

This is fear mongering by a Democratic partisan; nothing more, nothing less!