WASHINGTON — As President Obama prepares to visit Israel next week, he is turning, as he often does, to Benjamin J. Rhodes, a 35-year-old deputy national security adviser with a soft voice, strong opinions and a reputation around the White House as the man who channels Mr. Obama on foreign policy.

Mr. Rhodes is drafting the address to the Israeli people the president plans to give in Jerusalem, but his influence extends beyond what either his title or speechwriting duties suggest. Drawing on personal ties and a philosophical kinship with Mr. Obama that go back to the 2008 campaign, Mr. Rhodes helped prod his boss to take a more activist policy toward Egypt and Libya when those countries erupted in 2011.

Now that influence is being put to the test again on the issue of Syria, where the president has so far resisted more than modest American involvement. After two years of civil war that have left 70,000 people dead, Mr. Rhodes, his friends and colleagues said, is deeply frustrated by a policy that is not working, and has become a strong advocate for more aggressive efforts to support the Syrian opposition.

Administration officials note that Mr. Rhodes is not alone in his frustration over Syria, pointing out that Mr. Obama, too, is searching for an American response that ends the humanitarian tragedy, while not enmeshing the United States in a sectarian conflict that many in the White House say bears unsettling similarities to Iraq. Three former officials of the administration — Hillary Rodham Clinton, Robert Gates and David Petraeus — favored arming the opposition, a position Mr. Rhodes did not initially support.