WASHINGTON - The toxic relationship between America's political parties has spilled onto distant, perilous terrain: a Middle Eastern argument over nuclear weapons.
There's nothing new about Democrats and Republicans thwarting each other at home — smothering routine legislation with filibusters and vetoes that have made this the least productive congressional period in generations.
Now their spat has seeped into Iran.
Republicans sent the Iranian government a letter Monday warning that any international deal on nuclear development could be undone by President Barack Obama's successor. That's after they invited Israeli Prime Minister Benjamin Netanyahu to campaign against a deal from a congressional podium.
Predictably, Democrats fumed about sabotage.
"To throw sand in the gears here is not helpful and is not, frankly, the role that our Founding Fathers envisioned for Congress to play when it comes to foreign policy," said Obama spokesman Josh Earnest.
"It's surprising to me that there are some Republican senators who are seeking to establish a back channel with the hardliners in Iran to undermine an agreement between Iran and the broader international community."
The main author of the letter said it was important to help Iranians to understand the U.S. Constitution, and know that any treaty not approved by Congress could easily be ripped up by a future administration.
Written by Sen. Tom Cotton and signed by the vast majority of Senate Republicans, the letter said: "President Obama will leave office in January 2017, while most of us will remain in office well beyond then — perhaps decades."
And the partisan finger-pointing persisted throughout the day.
Later Monday, a poll pointed to a deeper issue. An NBC survey showed the extent to which Americans view their politics as polarized: 89 per cent said the president and Congress stick to their positions and don't work together.
A longtime White House staffer shared some of the president's views on that phenomenon in a less-noticed piece of political news Monday.
He said the president mostly gave up trying to work with Republicans around 2011. That's when the U.S. risked a default on its debt, because the president and Republicans couldn't get a budget deal.
Dan Pfeiffer said the president now believes structural forces are at play in U.S. politics that punish compromise.
U.S. politics, he said, rewards those who cater to their own base. As an example, he said the president gets a political bounce whenever he does something progressive — impose immigration reform by executive order, support same-sex marriage, or introduce climate rules without Congress.
"The incentive structure moves from going after the diminishing middle to motivating the base," the longtime senior aide said in an interview with New York magazine, as he left his White House job after six years.
Pfeiffer cited three reasons the parties can't work together much these days:
—The torrent of money flowing into U.S. politics from ideological and special-interest groups. The money, he said, tends to go to people who fight for a cause, not compromise with the other side.
—A more partisan news media. The CNN model has splintered into an ecosystem of proudly biased new outlets like Fox and MSNBC, which can create separate echo chambers.
—A historic, decades-long voting shift called "The Great Sorting." Legislative deals used to be worked out in the centre by two groups of political oddballs: left-leaning Republicans like the old northeastern Rockefeller types, who would work with the old conservative, mostly southern Democrats known as blue dogs.
That centrist species is now mostly extinct.
A study by Pew Research backs up Pfeiffer's assessment. It found that in the early 1970s more than half of Congress — 269 members — were creatures of the centre, which Pew defined as being ideologically located somewhere between the most conservative Democrat and most liberal Republican.
There were only 76 centrists by 1983, 12 by 1993 — and zero by 2011.
Now in that space where conversation used to exist, there's a gap. Research suggests the party that's shifted the most is the Republican party, toward the right.
Democratic Senate leader Harry Reid certainly blamed them in the Iran spat: "Republicans don't know how to govern, but they do know juvenile political attacks. They're undermining our commander-in-chief out of spite," he tweeted.
But a few people noted Reid earned a reputation for systematically refusing to hold votes on bills put forward by Republicans, notably the Keystone XL pipeline from Alberta.
The pipeline vote eventually illustrated that partisan divide: Almost 100 per cent of Republicans supported it. Only 16 per cent of Democrats in Congress voted for it. In reality, American public opinion is far less polarized than that.
Canadian officials have tried to prevent Canadian oil from falling into that partisan gulf and will involve at least one Democrat in any pro-Keystone event in Washington.
Last week, Netanyahu also said he wanted Israel to remain a bipartisan issue. But his speech had Democrats fuming, and Republicans offering standing ovations and signing their names in a letter to Iran.