WASHINGTON — Europe’s economic crisis could send shock waves roaring across the Atlantic that would drag down the fragile U.S. economy and threaten President Barack Obama’s hopes for a second term. The president demonstrated Friday just how deeply he’s worried about that — and how little he can do to prevent it.

Obama used an impromptu news conference to prod European leaders to quickly and vigorously deal with their crisis. Along with that rare cross-Atlantic jawboning, he accused congressional Republicans at home of holding back a U.S. rebound.

The president held forth, unprompted, on what European leaders could and should do to fix their economic woes, though he insisted he was not “scolding them or telling them what to do.”

He never mentioned his election opponent, Republican Mitt Romney. But the campaign seemed to be an important factor in the day’s events.

Powerless to take on the economic mess overseas by himself, Obama tried to show Americans he was nonetheless engaged in trying to help by offering ideas and advice. At the same time, he was sending a message to his European peers to be resolute and move firmly.

“Now, the good news is there is a path out of this challenge,” Obama said. “These decisions are fundamentally in the hands of Europe’s leaders, and, fortunately, they understand the seriousness of the situation and the urgent need to act.”

Weighing in late Friday on the U.S. economy, the rating agency that downgraded the government’s long-term credit last year renewed its assessment, asserting that leaders aren’t addressing the federal debt burden. Still, Standard & Poor’s said the United States has an “adaptable and resilient” economy and many other governments hold dollar reserves, a sign of confidence.

Demonstrating his limited direct influence in Europe at the same time he’s being thwarted at home by Republicans on his domestic agenda could reinforce a sense of presidential powerlessness — not an image an incumbent seeking re-election wants to project.