President Joe Biden’s choices and challenges on Ukraine were already extremely tough. As the war’s horror mounts, they are about to get even more excruciating.

Since Russia launched its onslaught last month, Biden has sought to punish and isolate President Vladimir Putin and to mitigate the slaughter of civilians by providing defensive weapons to the Kyiv government. But he’s also calibrated his actions to avoid being dragged into a dangerous direct conflict with nuclear-armed Russia while finessing his own delicate political situation at home.

As Putin escalates his assault, Ukrainian President Volodymyr Zelensky gets more desperate and the civilian toll becomes more appalling by the day, Biden’s balancing act becomes much harder.

The political heat on the President, after a period of unusual unity in Washington, is also about to rise. That will especially be the case if, as appears increasingly likely, the rest of the world is forced to watch an inhumane Russian siege and bombardment of Kyiv.

In a big Washington moment on Wednesday, Zelensky will deliver a virtual address to Congress. If his recent speech to the UK parliament, which drew Churchillian comparisons, is any guide, it will be a searing and inspiring rallying cry for lawmakers. If the Ukrainian President includes last-ditch pleas for fighter jets and a no-fly zone over his country, which Biden scotched on the grounds they could trigger war with Moscow, he will create extreme domestic pressure on the President.

Biden’s problem is that after unleashing full-bore economic warfare on Russia with extraordinarily tough sanctions, there are now limits to the steps he can take to significantly turn up pressure on Putin without risking a direct military or cyber conflict. Some of the President’s critics in Congress and in parts of the foreign policy establishment, including in his own party, argue that he’s been too cautious.

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