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The longstanding Vandenberg principle that foreign policy stops at the water’s edge has been eroding in recent years, but this week it was washed out to sea by three U.S. senators who plunged into the middle of a diplomatic mission by the secretary of state to undermine his efforts to negotiate Israeli-Palestinian peace.

Kerry was in Jerusalem on his 10th trip in less than a year trying to keep alive the sputtering peace talks he initiated six months ago. He praised Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas for their “important … courageous … difficult decisions” in pursuit of peace.

Netanyahu was in a bad mood. He used the welcoming ceremony to lash out at Abbas. Israelis have “growing doubt” about his commitment to peace, Netanyahu said, accusing the Palestinian leader of having “glorif(ied) the murderers of innocent women and men as heroes.” Abbas’ Palestinian Authority, he said, “continues its unabated incitement against the State of Israel.”

The private session with the secretary of state was even more strained. Netanyahu arrived for their Jan. 2 meeting “angry and made sure his guest knew it,” reported Barak Ravid in the daily Haaretz. “[T]he atmosphere was ugly and tense,” and it took the prime minister more than an hour to calm down, he wrote.

Netanyahu next briefed three visiting Republican senators — John McCain of Arizona, Lindsey Graham of South Carolina and John Barrasso of Wyoming — who emerged from that meeting in full Likud attack mode, sounding more like spokesmen for the Israeli prime minister than members of Congress.

They went after American foreign policy not only on the peace talks but also Syria (they want direct military intervention), Iraq (they wish we were still there) and Iran (do whatever Bibi wants).

Gone was any semblance of observing the longstanding tradition of bipartisanship on foreign trips and keeping politics out of congressional missions overseas.

“Prime Minister Netanyahu has serious, serious concerns about the plan that has been presented to him — whether it be the ability of Israel to defend its borders, the viability of a Palestinian state and their intentions and their actions toward the State of Israel, and particularly on the overall security — whether it’s boundaries, areas under Palestinian control,” said McCain.

Israeli Prime Minister Benjamin Netanyahu shakes hands with Secretary of State John Kerry in Jerusalem on Jan. 2. photo/jta-flash90-gpo-amos ben gershom

Foreign policy professionals from Congress, as well as former diplomats and others I spoke to, all told me this was unprecedented and, said one, “sure as hell bad.”

Chris Nelson, a former congressional foreign affairs staffer and veteran analyst, called this “a truly stunning betrayal of principal, tradition and practice in legislative involvement in foreign policy.” The three senators were “deliberately undercutting” Kerry and “supporting hard-line Israeli elements long opposed to anything from the Obama administration.”

A former State Department official told me it was standard procedure to discourage congressional delegations from being in the country at the same time as the secretary of state. There may be some overlap, as one arrives and the other leaves, but it is highly unusual for American lawmakers to show up during a diplomatic mission by the secretary of state and not only criticize American policy but sound like they are speaking for the foreign government.

The Vandenberg principle was first articulated by Michigan Republican Sen. Arthur Vandenberg at the start of the Cold War. He abandoned his longstanding isolationism to back President Harry Truman and the creation of NATO. It was essential, in his view, that America show strength and unity, and no politician should go abroad and undercut any president of any party. It was called a monument of bipartisanship, but that is something verboten in today’s Congress.

The principle has been dying in recent years, but these three amigos trampled it to death.

McCain and his colleagues also echoed Netanyahu’s views by calling for tougher new Iran sanctions — in violation of last November’s Geneva agreement that included a six-month freeze on such legislation to give the nuclear negotiations a chance to succeed.

McCain said Obama is not being tough enough on Iran. He may be right on both counts, but his time and venue make his motives suspect.

Graham sounded like the spokesman for the national camp and settler movement that oppose any territorial concessions or Palestinian statehood. “Once you withdraw, the ability to go back is almost impossible,” he warned. “I really do believe that the idea of withdrawing has to be considered in light of Gaza.”

They didn’t stop there. McCain also accused the Obama administration of not doing enough to curb the violence in Syria, which he said also endangers Israel. He and Graham have urged military intervention in that war, something broadly opposed in Congress and among the American public. They also blamed the Obama administration for the “tragic” and “predictable” capture of Fallujah, Iraq, by al Qaeda forces. The pair opposed the U.S. withdrawal from Iraq two years ago.

The peripatetic senators only make Kerry’s job more difficult and peace more elusive. Their message to Netanyahu is they will run interference for him and block anything the administration wants, and to Abbas they’re saying, we’re Bibi’s boys so you can fuggedaboutit.

When Kerry next returns to the region, will Bibi’s boys be there, too?

Douglas M. Bloomfield is the president of Bloomfield Associates Inc., a Washington, D.C., lobbying and consulting firm. He spent nine years as the legislative director and chief lobbyist for AIPAC.