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@TaxGuyBill wrote:
FICA is Chapter 23.

For purposes of FICA, the person is still a Nonresident Alien (until they meet the requirements to become a Resident Alien without the election).

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For the sake of clarity and for others who may come across this post in the future, FICA is under Chapter 21, not Chapter 23 (which is for FUTA).  FICA liability is also not determined by whether an individual is a US person based on §7701(b).  In fact, FICA is assessed on wages as defined in §3121(a), pursuant to §3101.

Except as provided for under §861(a)(3), compensation for labor or personal services performed in the US is US source income, which is subject to FICA.  In other words, FICA is assessable on foreign nationals who may be deemed an NRA under §7701(b), whether or not the compensation is paid by or on behalf of a US or foreign employer.

There are, however, various exceptions (this is tax law we're dealing with after all).  One of which is where totalization agreement's detached worker rule applies.  Another is the carve out in §3121(b)(19) for services performed by F, J, M, and Q visa holders so long as they remain an NRA and those services are performed to carry out the purposes of their respective visas.