Immigration and security, the hot-button topics so far, are expected to feature prominently in the clash from 8:15pm along with US-European tensions and the woeful state of the German economy.
Despite not being in the room, the resurgent far-right Alternative for Germany (AfD) may again steal the show, particularly after US President Donald Trump's administration waded into the campaign with strong backing for the party.
The AfD has surged to second place in the polls ahead of Sunday's vote, scoring around 20 percent - second only to Merz's CDU/CSU alliance, which is polling around 30 percent.
Even if the conservatives win as expected, Merz would need to find coalition partners - a process that could take weeks and has been complicated by the rise of the AfD and smaller parties that could make it into parliament.
READ ALSO: With Merz in charge, foreigners should expect volatile months ahead
Merz continues to insist, despite relying on the AfD's support in a parliamentary vote last month, that he would never form a coalition with the anti-immigration party.
In a survey by polling firm INSA published in tabloid Bild, 40 percent of those quizzed considered Merz more competent overall than Scholz, who was favoured by just 28 percent.
Merz scored better than Scholz across a range of flashpoint election issues, including immigration, security and the economy, an outcome the right-leaning Bild labelled a "severe poll damper for Scholz shortly before the election".
READ ALSO: Scholz vs Merz - what we learned from Germany's 'pretty boring' election debate
Another area of tension with the United States could feature in the debate - Washington's shock policy shift on the Ukraine war after Trump administration officials met their Russian counterparts in Saudi Arabia on Tuesday.
The talks have blindsided Ukraine's European allies, including Germany, with Scholz warning against forcing a "dictated peace" on Kyiv.
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