German opposition leader Peer Steinbrück said on Sunday that he would not enter into a “grand coalition” with Chancellor Angela Merkel’s conservatives as the junior party, raising the possibility of a right-wing victory in September’s election.

Steinbrück, who was Merkel’s finance minister in the last such grand coalition between 2005 and 2009, told ZDF television that a government of Christian and Social Democrats was unlikely. “We all know what happened last time around,” he said, referring to support for his party plunging to 23 percent in the 2009 election from 34 percent in 2005.

Earlier in the day, Gregor Gysi of the far-left Die Linke had called on Steinbrück to consider forming a government with his party as well as the Greens who are expected to win between 13 and 15 percent support next month, replacing Merkel’s liberal coalition party as the third largest in parliament.

“I don’t see any reason that would prevent Die Linke from becoming part of a government ruling in Germany one day,” Gysi told the Bild am Sonntag newspaper which published a poll that put Steinbrück’s Social Democrats, their Greens allies and the Left at 46 percent, a point ahead of the ruling parties. “Without us, they’re not going to win the chancellery,” Gysi added.

The Greens and Social Democrats have shunned Die Linke because it is the successor to communist East Germany’s ruling socialist party and seeks to withdraw Germany from NATO. Polls predict that its support will drop from nearly 12 percent in the last election to 8 or 9 percent next month.

The Social Democrats have quietly dropped their ban on alliances with Die Linke at the state level in former East Germany, however, where it remains popular. Between 2010 and 2012, the party also backed a coalition government of Greens and Social Democrats in North Rhine-Westphalia when the two fell short of a majority.

Merkel’s current finance minister Wolfgang Schäuble suggested earlier this year that the opposition parties should repeat that experiment if the conservatives and liberals failed to win reelection. In a television interview last month, the chancellor also made that suggestion and warned that the only alternative to a continuation of her government was an all left alliance — scaring moderate voters who might otherwise consider voting for the Social Democrats.

Few Germans favor including Die Linke in a federal administration. If Steinbrück, whose party is around 25 percent support in the polls, were to declare his intention to form a government with them ahead of the election, he could lose centrist voters.

Despite their protestations, the Frankfurter Allgemeine business newspaper reported on Sunday that the Social Democrats would be willing to consider ruling with the conservatives again, provided Merkel didn’t return as chancellor. Since the liberal Free Democrats are struggling to cross the 5 percent election threshold, it might be the only realistic outcome.