WASHINGTON—President Trump finds himself a “yes” away from brokering a cease-fire between Russia and Ukraine. But it is Russian President Vladimir Putin who has to say it, and he is leaning in the other direction.

Russia’s stalling contrasted with Trump’s rush for a valuable breakthrough as his trade policies set off stock market slumps and cast a cloud over the economy. “I think we’re going to be in very good shape to get it done. We want to get it over with,” Trump said at the Oval Office on Thursday, in an outburst of optimism that jarred with reality.