It is not surprising that, according to opinion polls, Ukrainians consider the United States, along with Poland, our primary ally in the world.

And so it is bitter to learn that under Donald Trump, it appears that the leadership of the United States has now joined the list of those who would use Ukraine to pursue their own narrow ends, and do so in ways that hinder our own efforts to improve our country. It is especially painful that this is happening at a moment when the new Ukrainian leadership desperately needs a functional partnership with Washington.

With the victory of Volodymyr Zelensky in this year’s presidential and parliamentary elections, Ukrainians got one more chance at fighting corruption by making further reforms to the judiciary and the prosecutor general’s office. Mr. Zelensky, a former comedian who won more than 70 percent of the vote, has promised not to run for a second term. Some of his first steps have been encouraging: He appointed a new professional and trusted team at the prosecutor general’s office — an especially important change, given how often prosecutions have been wielded as political weapons in Ukraine — and he has empowered anti-corruption bodies.

But the prospects for rule of law and a corruption-free administration in Ukraine remain fragile; many are concerned, for instance, about the influence of oligarchs on Mr. Zelensky’s agenda.

And Ukraine remains in a precarious position. Crimea is still under Russian control, and the war in the east continues; the fight against corruption at home is also not yet won. In all of these matters, Ukraine is reliant on bipartisan American support, which is why our leadership is desperate to avoid becoming part of the American presidential campaign.