Could Congress pass a law requiring all people who live here or visit here to have a microchip implanted under their skin for ID?

They could.

But I can't imagine any Supreme Court would uphold such a law as Constitutional, but so long as it is just an option (and not compulsory), then maybe something like that would happen. The key difference is whether one is compelled to do it, or one is offered the choice to do it (or not).

Edit:

Maybe I should clarify that I don't particularly support such an idea, I am just stressing the difference between something being mandatory or optional.

I suppose they could, but what would that accomplish (other than spawning a black market in forged microchip implants)?

Why would they need to do that... we already have smartphones

- Apple, Google, Facebook etc knows all about us.

Theoretically, Republicans might push this government ID concept that far.

They would have a hard time getting that one past a court.

Yes, they could pass that law. But it would probably be struck down in the courts as a violation of the Fifth Amendment. Moreover, since the majority of people would never support such a law, it's unlikely Congress would pass it.

How about a 666 tattooed on their foreheads?

The governments not getting me

We started doing that with a vaccine where I live.

The government is already trying to do with with anyone that's born in this country.