Ross was a powerful asset for the campaign. He went to synagogues in swing states and participated in conference calls to tell Jewish voters that he was supporting Obama over Clinton’s first lady. He continued his work with the campaign during the general election, which Obama won with between 74 and 78 percent of the Jewish vote. (Democrats typically get at least 70 percent of the Jewish vote in presidential elections. The best any Republican has ever done with the Jewish vote was Ronald Reagan, who won 40 percent in 1980.)

This year, Obama is again mired in similar territory, but now he won’t have the benefit of Ross koshering his record with pro-Israel voters. Despite unprecedented levels of military cooperation with Israel, the president is being attacked by the right for a rocky relationship with Israeli Prime Minister Benjamin Netanyahu, whom he has pressured to stop settlement construction in the West Bank and housing construction in East Jerusalem. And instead of anonymous emails, Obama’s record on Israel is being attacked on the Web and in television ads paid for by unregulated and at times anonymous independent expenditures financed by big-money donors like casino mogul Sheldon Adelson.

Once he joined the Obama White House, Ross was known to be a critic of some of the administration’s policies. In 2009 and 2010, Ross disagreed with the foreign-policy team’s strategy to distance the president from Israel in order for the United States to have more credibility in the peace process. In an interview this month with The Washington Post, Ross said he thought Obama’s initial push for Israel to freeze settlement activity in the West Bank and East Jerusalem as a condition for Palestinians to participate in peace talks was a mistake.

Ross’s views were well-known to the pro-Israel lobby in Washington, and he was often trotted out to relieve concerns the Jewish and pro-Israel community had with Obama’s initial approach to the Jewish state. Others, however, speculated that Ross was frustrated that the policies he pursued under Obama have not yet gotten results.

“Dennis is about doing things,” said Aaron Miller, who was Ross’s deputy on the peace process during the Clinton years and is now a scholar at the Wilson Center, a public-policy think tank in Washington, D.C. “The peace process is stuck and is likely to remain stuck. The fact is no amount of hand-holding is going to assuage the concerns and suspicions of a pro-Israel community which has now seen some of its fears realized. It may well be that this is the other piece of this. I wouldn’t want to try to sell Obama to the Jewish community in this environment.”

Alan Solow, a national co-chairman of the Obama campaign who often accompanied Ross on talks with synagogues and other Jewish groups, said Ross was still a supporter of the president.

“Dennis left the administration for the reason he stated—a promise to his wife—and not because of any disagreement with the president,” Solow said. “He was an important contributor to many of the president’s policies regarding Iran and the Middle East and I have every reason to believe that he continues to support the path being pursued by the president.”

For now, the Obama campaign is continuing its outreach to Jewish voters without Ross. An Obama campaign official told The Daily Beast it often uses Debbie Wasserman Schultz, chairwoman of the Democratic National Committee, and former Florida congressman Robert Wexler as surrogates with Jewish voters. The White House also conducts its own Jewish outreach with senior Obama-administration officials like Jack Lew, the president's chief of staff. This week, Bloomberg first reported that Haim Saban, an American television producer and billionaire, had donated to the pro-Obama super PAC Priorities USA Action. Saban publicly criticized Obama in 2011 for airing out disagreements with Israel in public.