So theoretically if any one of the party in american system gets 2/3rd people of the same party and they all agree to one person and try to amend the constitution itself to add whatever they want, can't they theoretically create a purest form of dictatorship (one which lasts forever instead of just 5 years)

I mean given how much is already happening in America, I am just curious from a legal standpoint if there could be done something like that (forgetting the insane backlash but still), what could the president of america do to completely sieze the constitution ?