Roesler proclaimed Sunday "a great day" for his party.
Merkel also has profited so far from stumbles by Steinbrueck, a former finance minister who drew criticism in recent weeks for saying that the chancellor earns too little — adding to the controversy over his own high earnings from speeches.
His party improved a little on its feeble performance five years ago in Lower Saxony, polling 32.6 percent. Its allies, the Greens, made bigger gains, to 13.7 percent. That gave them 49 and 20 seats, respectively, in the state legislature in Hannover; the CDU took 54 and the Free Democrats 14.
Opposition leaders argued that their overall gains and the losses for the center-right showed that a change to a center-left government is possible in Berlin, despite unpromising recent polls.
Steinbrueck conceded that his recent troubles hadn't helped, though.
"I am well aware that there was no tailwind from Berlin, and I am also aware that I bear a certain share of responsibility for that," he told supporters. He said he felt "a certain amount of relief" at the result.
Two parties that have drained support from the center-left over recent years, the Left Party and the Pirate Party, failed to get enough votes Sunday to enter Lower Saxony's legislature.
A change of government in Lower Saxony will give opposition parties a majority in Parliament's upper house, which represents Germany's 16 states.
Though that has limited practical effect, it would enable the Social Democrats and Greens to showcase their plans for Germany by sending policy initiatives to the lower house — controlled by Merkel's coalition — on issues such as their call for a mandatory national minimum wage.
However, it remains to be seen whether they will be able to build up momentum and prevent Merkel keeping power — possibly by finding a new coalition partner.
With eight months left before the national vote, "so much can change that I'd just say we should be careful about projecting such results onto the national level," Lothar Probst, a political scientist at the University of Bremen, told Phoenix television.