American politics once stopped at the water’s edge. Now, it doesn’t even slow down.

By trying to deter Iran’s leaders from cutting a nuclear deal with the U.S. and Europe, 47 Republican senators this week caused the most serious rupture in the tradition of bipartisan foreign policy in at least a generation.

“We’re in new territory here,” said David Rothkopf, author of a book on the National Security Council. “There’s been an escalation in the role partisanship is playing, and it’s become particularly destructive.”

Democrats -- and even some Republicans -- say a lack of unity confuses allies and adversaries alike, making the U.S. appear to be an unreliable negotiating partner.

“This hurts America,” Sandy Berger, national security adviser to President Bill Clinton, said of the Republicans who wrote a letter to Iranian leaders warning against an agreement. “This damages our strength in the world, damages the credibility of the president as he’s negotiating and therefore the next president who negotiates.”

The letter came a week after Israeli Prime Minister Benjamin Netanyahu assailed the U.S. administration’s Iran policy in a speech before Congress, having been invited to do so by Republican House Speaker John Boehner. Recent weeks have demonstrated that foreign policy has become just another partisan piñata, buffeted by one-sided media outlets and office-holders.

“I’ve never seen it as bad as this,” said former Republican Senator William Cohen of Maine, who served as Defense secretary in the Clinton administration. “It used to be the rule that politics stopped at the water’s edge.”

That hasn’t been entirely true for some time. The bipartisan Cold War consensus that united Democrats and Republicans against communism began to break down in the late-1960s as doubts arose over the Vietnam War. By the war’s end, that shared view of the U.S. global role was increasingly open to debate.

‘Teheran Tom’

Senator Tom Cotton, the Arkansas Republican who organized the Iran letter, has publicly said his goal is to wreck President Barack Obama’s prospects for a deal with Iran.

Representative Jared Polis, a Colorado Democrat, mocked Cotton on Twitter Tuesday as “Teheran Tom” and said the Republicans’ letter amounted to asking Iran’s Revolutionary Guard for “help in battle against U.S. diplomats.”

Washington’s polarization mirrors that of the public, according to a new McClatchy-Marist poll. Just 8 percent of Republicans said they approve of Obama’s conduct of foreign policy compared with 68 percent of Democrats.

The survey of 1,253 adults was conducted March 1-4.

Republicans say the president brought this on himself with his aversion to policies he inherited from his predecessor, George W. Bush, and his failure to consult Congress before pursuing the Iran talks.

Obama campaigned on a promise to withdraw U.S. troops from Iraq and Afghanistan. Republicans have chafed over what they regard as the administration’s continual effort to define itself as the un-Bush, according to one senior Republican who worked in the Bush White House.

Dissenting Republicans

Even some Republicans thought the letter went too far. Senator Bob Corker, the chairman of the Foreign Relations Committee, said, “For me, the better role is to try and seek bipartisan agreement that causes us to be able to appropriately weigh in on this deal.”

The senators’ letter was an extraordinary case of congressional intervention in a foreign policy negotiation. At the same time, though, unquestioned consensus has cost the U.S. in the past.

The Gulf of Tonkin Resolution that began the costly American military involvement in Vietnam passed the House of Representatives by a 416-0 vote in 1964. That divisive conflict cost 58,220 American lives, along with $738 billion in inflation-adjusted dollars.

Less Rancor

Cohen, who began his Senate career in 1979, recalls an era of greater substantive debate and less personal rancor. The Senate was populated with figures such as Democrats Henry “Scoop” Jackson of Washington and Sam Nunn of Georgia, while the Republican ranks included Jacob Javits of New York and John Warner of Virginia.

The 1991 debate over authorizing a U.S. attack on Iraqi forces occupying Kuwait featured a robust debate capped by a narrow 52-47 win for President George H.W. Bush.

“We had a vigorous debate back then,” Cohen said. “And it wasn’t partisan in the sense of the personal animosity that exists today between Congress and the president.”

That may be a rose-tinted view of history. Partisan meddling in foreign policy isn’t unprecedented. In 1968, Republican presidential candidate Richard Nixon dispatched Anna Chennault, a top supporter, to tell the South Vietnamese government to pull out of peace talks until after the November election.

Nixon promised the U.S. ally a “better deal” than what President Lyndon Johnson was offering.

Johnson learned of Nixon’s activities and called Republican Senator Everett Dirksen of Illinois to tell him to stop, according to a recording of the conversation on the website of the University of Virginia’s Miller Center.

“They oughtn’t to be doing this,” Johnson told Dirksen. “This is treason.”

No Monopoly

Republicans don’t have a monopoly on congressional interference. In April 2007, then-House Speaker Nancy Pelosi of California traveled to Damascus for talks with Syrian President Bashar al-Assad while Republican President George W. Bush was trying to isolate the Syrian government.

Bush criticized the visit for sending “mixed signals” to “a state sponsor of terror.”

That episode came more than two decades after congressional Democrats passed legislation barring the use of U.S. government funds to overthrow the Marxist government of Nicaragua. President Ronald Reagan subsequently used the proceeds of arms sales to Iran to support Nicaragua’s Contra rebels, leading to the Iran-Contra scandal.

The Obama administration also has brought an excessive concern for politics to its foreign policy deliberations, according to Robert Gates, who served as defense secretary under both Bush and Obama.

In Obama’s first term, during deliberations over sending additional troops to Afghanistan, Gates wrote that then-Secretary of State Hillary Clinton acknowledged that her 2007 opposition to the Iraq troop surge had been “political” because she was opposing Obama in the Iowa primary.

‘Remarkable’ and ‘Dismaying’

Gates wrote that he found the statement “remarkable” and “dismaying.”

To be sure, there are occasional traces of the old bipartisan spirit. In January, when Obama visited Saudi Arabia to meet with the kingdom’s new ruler, he was accompanied by leading foreign policy figures from both parties, including Republican former secretaries of state James Baker and Condoleezza Rice and former national security advisers Brent Scowcroft and Stephen Hadley.

Still, the current environment is a far cry from the Cold War era inaugurated in 1945 by Republican Senator Arthur Vandenberg. The Michigan isolationist endorsed the Truman administration’s policies to rebuild Europe and prevent communist domination of Greece and Turkey.

Anti-Communist Unity

For a generation, Republicans and Democrats alike took for granted a mainstream view that the U.S. would counter -- with force if necessary -- any expansion of communist influence.

“That was a strength of our foreign policy, really one of the great things about the post-strategy,” said Hadley, who was national security adviser from 2005 to 2009. “We did have a strategic consensus that spanned both parties, and over time multiple presidencies, Republican and Democrat.”

Partisanship may get worse before it gets better. With Islamic State, Russian aggression in Ukraine and the rise of China, foreign policy is likely to play a larger role in the 2016 campaigns than it did in recent elections.