Koch, a Democratic Jew, hoped his support would send a message to the White House that Jewish Americans were dissatisfied with Obama's policies on Israel -- namely the president's position that a future Mideast peace solution requires Israel to return to its pre-1967 borders, with land swaps.
"I like President Obama ... I helped get him elected," Koch said at Turner's election night party. "But he threw Israel under the bus."
His remarks were met by cheers from the crowd, including one man who shouted, "He's a sell-out!"
Koch's nod pivoted the race into a relentless competition over who was a stronger supporter of Israel: Turner, a conservative Republican, or Weprin, an orthodox Jew.
Minutes after the election, the Republican Jewish Coalition released a statement in support of Turner.
"Jewish voters are coming to see that Republicans offer real solutions to our economic crisis, are resolute friends of Israel, and represent a way forward to a better future," said executive director Matt Brooks.