IR masters student looking to do a PhD in Econ. What's the best way to meet the Pre-reqs?

Hi,

I’m from the US, but I did my undergrad and am currently working on my masters in England. Both degrees are in International Relations/Politics, but I’ve always been really drawn to economics and would rather pursue a PhD in Econ than IR.

As part of the curriculum for IR, I’ve taken a number of economics classes but I don’t think I know enough/ have the mathematical background to go directly to a graduate degree in economics.

If I do want to switch areas and pursue either a masters or PhD in economics, what is the best way to prepare? Are there any dedicated programs at good universities in the US?

My current University (Cambridge) has a “Post-graduate Diploma” program in economics. It’s essentially a 1 year accelerated course for people with degrees in other areas and covers all of the Pre-reqs for Econ masters here (plus most of the ones for a PhD).

I was really interested in doing that, but you can only be in the UK for a certain amount of time on a student visa without moving up to a PhD and I’m approaching the limit.

Does anyone know of anything similar in the US? I know you can enroll at a local university and take classes, but I really liked the idea of the diploma at Cambridge because it seems a lot more structured. Also if I decide not to do a PhD, I think “post-graduate diploma in economics” looks a lot better on a CV than “random year taking econ classs after completing my masters in another field”

Sorry for all the word vomit, but I’d really appreciate any insight people have!