Merz's proposal comes as the German economy experiences prolonged stagnation and US President Donald Trump's direct overtures to Russian President Vladimir Putin have left Europe feeling vulnerable on security.
The proposed constitutional changes to ease strict borrowing limits in the area of defence and establish a 500-billion-euro ($542-billion) fund for infrastructure were described as historic when they were presented last week.
However, Merz's conservative CDU/CSU bloc and their likely coalition partners, the centre-left Social Democrats (SPD), face a race against the clock to get changes through parliament.
With the parties still in talks to form a government following a general election last month, the constitutional amendments are being rushed forward to try to force them through the outgoing legislature.
If they can win the support of the Greens, the CDU/CSU and the SPD would have the two-thirds majority needed for constitutional changes in the current parliament.
After the newly elected parliament convenes at the end of the month, the far-right Alternative for Germany (AfD) and the far-left Die Linke will have more than a third of seats between them and will be able to block any such changes.
An initial debate on the amendments is scheduled for Thursday but senior Green party figures said they would recommend that their MPs do not support the changes.
Although the Greens were in favour of reforming Germany's strict constitutional spending limits, the changes proposed by Merz were not the ones needed, Green leader in the Bundestag, Katharina Droege, told reporters.
Merz and his partners from the SPD were creating a "treasure chest" to fund their political priorities, including tax cuts, Droege said.
"Whoever wants our approval for more investment must also show that it is really about more investment in climate protection, more investment in the economy in this country."
- 'Not unrealistic' -
Speaking shortly afterwards, CDU general secretary Carsten Linnemann said the party would hold talks with the Greens ahead of the parliamentary debate.
"I think it is completely legitimate for the Greens to say they have their own ideas," he said, adding that their counter-proposals were "constructive".
"They are not unrealistic and therefore I can imagine we can come to an agreement in these discussions," he said.
SPD parliamentary leader Lars Klingbeil, who has been heavily involved in coalition talks with Merz, said he "takes the Greens' concerns very seriously".
Klingbeil said he had been in discussions with Green leaders over the weekend and would meet them again with Merz on Monday night.
"I will not lose confidence that this can succeed," he said, describing the talks as his "top priority".
Germany's constitutional "debt brake", which has been the hallmark of Berlin's fiscal probity since it was introduce by former chancellor Angela Merkel in 2009, limits new borrowing to 0.35 percent of GDP.
- Ticking clock -
Merz's proposal would keep the rule essentially intact, while exempting any military spending over one percent of output.
The changes would also establish the fund for infrastructure to be spent over 10 years and ease spending rules for Germany's federal states and local government.
After the first debate on Thursday, the constitutional amendments are set to be voted on by MPs on March 18.
Although Germany voted in elections for a new parliament on February 23, the new legislature will not be constituted until March 25.
Green leader Droege suggested the conservatives reach out to the Die Linke, who have signalled a willingness to reform spending rules but have signalled greater scepticism about increasing defence spending.
Negotiations between the conservatives and the far left are unlikely to be straightforward, while Germany grapples with the urgency of increasing its defence spending.
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