Donald J. Trump has departed from Republican orthodoxy in multiple ways, but his consistently kind words for President Vladimir V. Putin of Russia stand as the most striking.

Mr. Trump’s rejection of traditional Republican stances on trade and immigration helped him win the presidential nomination by building a powerful base among blue-collar white voters who feel most aggrieved by economic and cultural change.

His embrace of Mr. Putin, however, fits no obvious political strategy. Staunch opposition to Moscow united the Republican Party throughout the Cold War.

The collapse of what Ronald Reagan called the Evil Empire only changed so much; four years ago, the Republican nominee, Mitt Romney, described Russia as America’s top geopolitical foe.