Ukraine has invited Florida Governor Ron DeSantis to visit after he said the Russian invasion was a "territorial dispute" with Russia which was not one of the US's "vital national interests".
Mr DeSantis is widely expected to run for president in 2024 and made his remarks in response to questions sent to possible Republican contenders.
Republican senators have lined up to criticise his views, too.
The US should not dismiss Russian aggression or war crimes, some said.
Mr DeSantis has not officially announced his intention to challenge Donald Trump for the Republican nomination, but has been taking all the necessary steps suggesting he will do so.
In his response to a questionnaire by Fox News anchor Tucker Carlson, he said:
"While the US has many vital national interests - securing our borders, addressing the crisis of readiness within our military, achieving energy security and independence, and checking the economic, cultural, and military power of the Chinese Community Party - becoming further entangled in a territorial dispute between Ukraine and Russia is not one of them."
Ukraine's foreign ministry spokesman Oleg Nikolenko tweeted his invitation to the top Republican on Tuesday.
"We are sure that as a former military officer deployed to a combat zone, Governor Ron DeSantis knows the difference between a 'dispute' and war," Mr Nikolenko said.
"We invite him to visit Ukraine to get a deeper understanding of Russia's full-scale invasion and the threats it poses to US interests."
Fellow Republicans were not impressed with Mr DeSantis's comments, either.
Lindsey Graham said this was "a misunderstanding of the situation" on the part of the governor.
"This is not a territorial conflict, it's a war of aggression. To say it doesn't matter is to say war crimes don't matter," Mr Graham said.
Senator Thom Tillis said this wasn't confined to Ukraine. "There's a humanitarian crisis. There are war crimes being committed."
And Senator John Thune thought "the majority of people in this country recognise how important it is, that Ukraine repel Russia".
President Vladimir Putin launched the full-scale invasion on 24 February 2022, saying Russia was intent on fighting Nazis in Ukraine.
His forces have razed entire cities and Russian troops are accused of numerous war crimes, which they deny.
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