It has a lot to do with their embrace of Trump and his admiration of Putin. Before that, they were largely 100% anti-Soviet and somewhat tepid about post-Soviet Russia. Putin has also expended a lot of resources courting US conservative influencers and political operatives during the 2010s, which eventually converged with the rise of Trumpism. There are of course still a lot of traditional establishment, Reagan type Republicans who correctly see Putin for who he actually is - an authoritarian tyrant. The likes of McConnell, Graham, Romney et al., are examples.
There's also an element on the left/right political fringes that is driven by domestic issues - for right wingers that's usually immigration and border security, for ultra-libs its usually a call to spend more on domestic social issues instead of on foreign countries. There's also an isolationist strain in the Republicans that calls for a severe reduction in foreign aid, as typified by the likes of Rand Paul.
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