BERLIN — Germany’s leading opposition party on Friday nominated a quick-witted former finance minister to challenge Chancellor Angela Merkel in next year’s general election, in a jolt to a race that had appeared heavily weighted in favor of the incumbent.

The former minister, Peer Steinbrück, 65, vowed in remarks after the announcement of his nomination to put the center-left Social Democrats in a “strategic position” to build a government with the Green Party, a clear rejection of the so-called grand coalition with Ms. Merkel’s center-right Christian Democrats that ruled from 2005 to 2009.

“I accept this challenge to win the next election with and for the S.P.D.,” Mr. Steinbrück said, using the German abbreviation for the Social Democratic Party. “We want to replace this government.”

The Social Democrats’ executive committee will convene on Monday to make Mr. Steinbrück’s nomination formal. Sigmar Gabriel, the party’s chairman, who had also been considered a possible candidate, said the leadership agreed two weeks ago to nominate Mr. Steinbrück after Frank-Walter Steinmeier, the party’s parliamentary leader, withdrew from the contest.