Related Question on Metaculus:

Depending on the US supplying Ukraine with an ATACMS, Will Russia use nuclear weapons against Ukraine before 2024?

The MGM-140 Army Tactical Missile System (ATACMS) is among the United States military's most advanced surface-to-surface long-range missile systems. Its range is up to 300km and can be transported via MLRS or HIMARS. According to AP News, Ukrainian President Volodymyr Zelenskyy has "repeatedly requested" to recieve ATACMS from the US.

US Department of Defense spokesperson Colin Kahl reported on August 24, 2022:

we have provided [Ukraine] with hundreds and hundreds of these precision guided systems, and the Ukrainians have been using them to extraordinary effect on the battlefield. It's our assessment that the most relevant Munitions for the current fight are the GMLRS

[...] It's our assessment that they don't currently require ATACMS to service targets that are directly relevant to the current fight.

US National Security Advisor Jake Sullivan stated in July 2022 the US' concerns that providing ATACMS to Ukraine would escalate the conflict:

There are certain capabilities that the president has said he is not prepared to provide. One of them is long range missiles, ATACMS, that have a range of 300 kilometers, because he does believe that, while a key goal of the United States is to do the needful, to support and defend Ukraine, another key goal is to ensure that we do not end up in a circumstance where we are heading down the road towards a third world war.