Its not about the US getting physically involved, most Republicans believe more should be done to help the Ukrainians fight, such as sending Migs.
Nearly half (45%) say Joe Biden has been too cautious in supporting Ukraine. Again, this is driven by partisanship (60% of Republicans vs. 35% of Democrats).
https://www.ipsos.com/en-us/news-polls/Americans-support-limited-intervention-Ukraine-prevent-escalation-Russia
I think after the recent polling showing Biden at 40% it’s best to not trust the polls anymore.
When you explain to people that sending the mugs would be considered by Russia as an act of war, people no longer support it.