President Joe Biden on Friday for the first time publicly apologized to Ukraine for a monthslong congressional holdup in American military assistance that let Russia make gains on the battlefield.

Biden met in Paris with Ukrainian President Volodymyr Zelenskyy, who appealed for bipartisan U.S. support going forward "like it was during World War II."

A day earlier, the two had attended ceremonies marking the 80th anniversary of the D-Day landings in Normandy, where Biden had drawn common cause between the allied forces that helped free Europe from Nazi Germany and today's effort to support Ukraine against Russia's invasion and Zelenskyy had been greeted with a rapt ovation.

"I apologize for those weeks of not knowing what's going to happen in terms of funding," Biden said, referring to the six-month holdup by conservative Republicans in Congress to a $61 billion military aid package for Ukraine. Still, the Democratic president insisted the American people were standing by Ukraine for the long haul. "We're still in. Completely. Thoroughly," he said.

The apology — and Zelenskyy's plea for rock-solid support akin to the allied coalition in WWII — served as a reminder that for all of Biden's talk of an unflagging U.S commitment to Ukraine, recalcitrance among congressional Republicans and an isolationist strain in American politics have exposed its fragility. And, although unremarked upon, the specter of Donald Trump's candidacy loomed over the discussion, as the Republican former president and the presumptive nominee has spoken positively of Russian President Vladimir Putin and sparked Ukrainian concerns that he would call for it to cede territory to end the conflict.

Zelenskyy pressed for all Americans to support his country's defense against Russia's invasion, and he thanked lawmakers for eventually coming together to approve the weapons package, which has allowed Ukraine to stem Russian advances in recent weeks.

"It's very important that in this unity, United States of America, all American people stay with Ukraine like it was during World War II," Zelenskyy said. "How the United States helped to save human lives, to save Europe. And we count on your continuing support in standing with us shoulder to shoulder."