Merz unveiled the plans last week, vowing his centre-right CDU/CSU bloc and the centre-left SPD -- in talks to form a coalition after February's general election -- would quickly push them through before the end of the current legislature.

Fraying Europe-US ties under President Donald Trump have fuelled calls for Germany, long dependent on the US security umbrella, to quickly boost military funding, while infrastructure spending is seen as a route to pull Europe's top economy out of stagnation.

Pledging to do "whatever it takes", Merz has proposed exempting defence spending from the country's strict debt rules when it exceeds one percent of GDP and setting up a 500-billion-euro ($545-billion) fund for infrastructure investments.

While the plans have won praise from German allies abroad, who grew weary of inaction under outgoing Chancellor Olaf Scholz, Merz faces a desperate scramble to push them through parliament.

The plans needed to be pushed through rapidly due to the "dramatic changes in the global situation", Thorsten Frei, a senior lawmaker from Merz's CDU party said Thursday.

"We have to move very quickly now - the pressure is on, time is running out."

As the measures involve a change to the constitutionally enshrined "debt brake", which limits government borrowing, they require a two-thirds majority in parliament.

This means that the CDU/CSU and SPD want to get them passed before a new parliament convenes later this month in which far-right and far-left parties, who have expressed scepticism about extra defence spending, will be in a position to block the measures.

- Greens unhappy -

The Bundestag will convene for two special sessions so lawmakers can debate the plans, on Thursday and next Tuesday -- when a vote on the proposals is also to take place.

The debate is due to start around midday (1100 GMT), with Merz, lawmakers from the SPD, Green party and far-right Alternative for Germany (AfD) due to speak.

Merz's task has got trickier after the Green party, whose votes are needed to reach the two-thirds mark, threatened to torpedo the plans unless more is included in them for climate protection.

The CDU, SPD and Greens have since been locked in intense talks to hammer out a compromise.

However Green leader in the Bundestag, Katharina Droege, said Thursday she did not share the "optimism" of the other parties that an agreement could be reached quickly.

There was as yet no "common solution, so it is still the case that the Greens will not agree" to these plans, she said in a TV interview.

Merz's plans also face another threat with both the AfD -- which came second in the election -- and the far-left Die Linke party having filed legal challenges at the constitutional court, arguing there will be insufficient time for consultations.

- Calls for swift action -

If Merz fails to get his plans through, observers fear he would lose momentum and his future government could face the same paralysis that beset Scholz's ill-fated, three-party coalition, whose November collapse precipitated last month's election.

The pressure has only increased on him in recent weeks as Trump has become increasingly hostile towards Ukraine and made overtures to Russia.

Still the incoming government would have options to boost spending if the current parliament fails to pass the plans next week, analysts said.

Once Merz becomes chancellor -- likely in late April -- he could suspend the debt brake by invoking an emergency, as the previous government did during the pandemic, although this would only be a stopgap.

Merz's conservative bloc and the SPD are also due to begin full-fledged negotiations on forming a coalition Thursday after concluding exploratory talks at the weekend.

The fate of the spending plans could also have a bearing on these talks, noted Der Spiegel news outlet.

A failure to push them through parliament "would possibly also put an end to the coalition negotiations," it said.