I get the feeling russia would stage the whole thing regardless. If Ukraine got ATACMS and used it to take out targets solely in Ukraine it wouldn't be long before russia gathered up enough ATACMS wreckage and planted it next to a burning Russian base near the boarder and claiming it was US supplied weapons that attacked russian territory.
Given the state of Russian military I don't think they want the US involved, it would be over in a matter of hours, the only thing they have to counter a proper army is nuclear deterrent.