Updated: Mar 11, 2022
Just before the Russian invasion of Ukraine, Trump spoke favorably of Vladimir Putin, calling him "smart and savvy."
Once the invasion began, however, Trump changed course, calling the Russian assault a "holocaust."
Trump did this in part because he failed to persuade members of his Republican Party that Putin was a good guy. As NPR notes, "Unlike other issues, from Medicare to trade, in which Trump has brought Republicans to his view, he has struggled to lead on his position on Putin and Ukraine."
In fact, many Republican members of Congress are just as keen as their Democratic counterparts to send weapons and other aid to Ukraine. Yesterday, there was strong bipartisan support in the House for a successful 13.6 billion aid-to-Ukraine bill.
Trump is not just swimming against the tide of Republican Congressional representatives, however. He's also battling public opinion in the US overall, including among his Republican voters.
A January 10-17, 2022 survey of the US adult population conducted by the Pew Research Center, for example, found that 49% of all respondents, regardless of their political affiliation, thought of Russia as a "competitor," while a further 41% thought of it as an "enemy." Only 7% considered Russia to be a US "partner." The overwhelming majority of the US public, in other words, saw Russia in antagonistic terms.
These findings had no real partisan edge to them. Thus, 39% of Republicans and Republican-leaning independents, compared to 43% of Democrats and Democratic-leaning independents, regarded Russia as an"enemy." Virtually identical percentages (50% and 49%, respectively) thought of Russia as a "competitor." These differences are small, given margins of error, Trump's pro-Russian statements, and the overwhelming partisan divide about so many other issues.
This means that even before the invasion, Trump faced an uphill battle convincing his supporters that Putin was a good guy.
Note, moreover, that this survey was taken before the Russian invasion and the 24/7 media coverage of civilian casualties.
We also have relevant post-invasion data. On March 4-6, Quinnipiac polled the US adult population and found bipartisan support for a vigorous US response. 82% of Democrats, 66% of Republicans, and 70% of Independents, for example, favored a ban on Russian oil. Overall, 80% of respondents overall favored a US military response if Russian forces attacked a NATO ally (this figure was not broken down by political affiliation).
These figures suggest that while Trump may have consistently identified and catered to his base's worldview over the past six years, he got it very wrong on Russia.
This, in turn, explains why he quickly changed course. Expedient as always, Trump has caught up to his base and is spinning in their direction.