The Chancellor also praised Laschet as a "bridge builder who can bring people together".
Merkel has wanted to keep a low profile during the election campaign and prepare for her resignation after 16 years as German leader.
But she has still been thrown into Armin Laschet's election campaign to strengthen his support.
"To get a stable Germany, Armin Laschet must become chancellor and the CDU and CSU must be the strongest force," she said.
At the same time in Potsdam, outside Berlin, the victorious Olaf Scholz chased the votes.
Here, two of the three top candidates in the election - the Social Democrat Olaf Scholz and the Greens' Annalena Baerbock - have their constituency.
The Scholz Social Democratic SPD has been topping opinion polls since August, and many have already calculated the main opponent, Armin Laschet from the CDU, who has so far failed to win the voters' favor.
However, it is moving - a recent survey from the opinion institute Allensbach shows that only one percentage point now distinguishes between the two political giants.
About four out of ten Germans have voted early this year, but there is still time to attract insecure voters.
Polling stations open at 8 a.m. on Sunday morning.