Germany’s centrist politicians are losing votes to the far right and the far left. They are losing faith in America, their longtime friend and protector.

And they are rapidly losing what could be their best chance to address both those problems at once.

The German government in waiting, led by the likely next chancellor, Friedrich Merz, introduced a raft of measures to a lame-duck session of Parliament on Thursday that it was billing as an urgent response to President Trump and his administration’s threats to withdraw American military support for Europe.

The package would rip away Germany’s signature limits on government spending and allow for hundreds of billions — or potentially trillions — of euros in new efforts to rebuild the country’s aging infrastructure and military.

But Mr. Merz and his center-right Christian Democrats, who narrowly won elections in February, do not yet have the votes to pass the proposed changes. Whether they can muster them over the next week will determine whether Germany is prepared to make a seminal strategic shift. It will also shape Europe’s fate as it confronts its most hostile security environment since World War II.

“Do you seriously believe that an American government will agree to continue NATO as before at the NATO summit in The Hague at the end of June if Germany and, together with Germany, the European NATO partners, are not prepared to take a new path?” Mr. Merz asked lawmakers in a fiery speech on Thursday.