I believe how it works is that if the letter is not responded to, it becomes a subpoena. If that's not responded to, it becomes either criminal or civil contempt of Congress. Bear in mind that this senator is chairman of the Senate Finance Committee.

https://legalclarity.org/what-is-the-penalty-for-refusing-a-...

https://www.senate.gov/about/powers-procedures/investigation...

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e44858 a day ago | parent [-]

Can a single senator issue a subpoena, or does it require a vote?

	▲	efitz a day ago \| parent [-]
		Some committees are chartered with the ability of the committee chair to issue subpoenas.