The recent downshift in the economy has dealt a powerful blow to President Obama’s chances of re-election in November. But perhaps most worrying for the president is just how little sway he retains over where the economy goes from here.

His attempts to convince voters as he stumps around the country that Europe’s financial mess and Republican obstruction are largely to blame for the faltering economy only underscore how his destiny hinges on decisions by other people, notably the German chancellor, Angela Merkel, and the Republican speaker of the House, John A. Boehner.

At home, the president’s plans to stimulate the economy with tax cuts and spending programs have invariably crashed against solid Republican opposition in Congress. As a result the government is becoming a big drag on the economy, cutting more than half a million jobs at the federal, state and municipal level since national employment bottomed out in February 2010.

In Europe, the political constraints are the same. Most economists, including those in the Obama administration, say they believe that for the euro to survive, weak European countries must grow. But this means curbing the harsh austerity policies imposed in return for aid from European institutions and the International Monetary Fund — policies that have only deepened the countries’ recessions. Germany, the only country in Europe with the wherewithal to backstop its poorer neighbors’ finances, has so far said no.