Armin Laschet, the Christian Democratic chancellor candidate in Germany's recent election, took responsibility on Saturday for the party's record-low vote share.
"The responsibility for this result falls to me as a leader and candidate for the chancellery. I am responsible for the electoral campaign," said Laschet at a party congress for young people in Münster.
Laschet became CDU leader at the beginning of the year and hoped to succeed Angela Merkel as German chancellor.
But the CDU received just 24.1% of the vote in the legislative elections on September 26, the worst result in their history and finished second behind the Social Democrats.
The party, which under Merkel has headed the government for nearly 16 years, could now become part of the opposition as the Social Democrats (SPD) discuss a governing coalition with the liberal Free Democratic Party (FDP) and left-leaning Greens.
Laschet said conservatives should prepare to join the opposition as a new coalition emerges, stating that "everything indicates that a traffic light coalition (referring to the SDP, FDP and Greens) will emerge."
German election: Traffic light coalition moves closer to forming a government
He added that the CDU needed more unity after the party's internal struggle and infighting over naming a candidate during the election.
"That is what we have to learn," Laschet said. "Any mayoral election will be lost if there's a decision that 'he or she is our candidate,' and afterward those who weren't chosen say 'Yes, but we had a better one.'"
"The Social Democrats showed how you can run a united election campaign despite contrasts," he added. "That was once the Union's strength, and it must in the future become the Union's strength once again...We must again learn this virtue of standing together if we want to win elections in the future."
Laschet plans to step down as president of the North Rhine-Westphalia region, the most populous in Germany and the vibrant heart of the CDU.