WASHINGTON (AP) - Relations between President Barack Obama and congressional Republicans have hit a new low.

There has been little direct communication between Obama and the GOP leadership on Capitol Hill since Republicans took full control of Congress in January. Obama has threatened to veto more than a dozen Republican-backed bills. And House Speaker John Boehner infuriated the White House by inviting Israeli Prime Minister Benjamin Netanyahu to address Congress without consulting the administration first.

But the dispute over Obama's high-stakes nuclear negotiations with Iran has put the relationship perhaps beyond repair.

The president and his advisers are seething over Republican efforts to undermine the sensitive discussions with Iran, most recently by sending an "open letter" to the country's leaders warning that any nuclear deal could expire the day Obama leaves the Oval Office. "I cannot recall another instance in which senators wrote directly to advise another country - much less a longtime foreign adversary - that the president does not have the constitutional authority to reach a meaningful understanding with them," Vice President Joe Biden, who spent nearly four decades in the Senate, said in an unusually harsh statement.

For their part, Republican lawmakers call their outreach to a hostile nation a reasonable response to an administration they say has spurned Congress and ignored its prerogatives at every turn. It's the starkest sign yet that Republicans see an adversary, not a potential partner, in Obama's White House - even on foreign policy issues where partisan differences have traditionally been somewhat muted.

"The mutual efforts to work together under this administration have just disappeared, so I think there's a sense now that extraordinary things occasionally need to happen to be sure that the president understands how strongly the Congress feels," said Sen. Roy Blunt, R-Mo.

The dismal state of the relationship has largely sunk the slim prospects for bipartisan cooperation in Obama's final two years in office, with one exception being work on international trade agreements that the White House and Republicans have long supported. And with Obama firmly eying his legacy, even his own advisers have conceded that a president who took office vowing to bridge partisan divides is virtually powerless to influence his political opponents.

"We don't have the ability to communicate with them," Dan Pfeiffer, Obama's recently departed senior adviser, said in an interview with New York magazine. "They are talking to people who agree with them, they are listening to news outlets that reinforce that point of view, and the president is probably the person with the least ability to break into that because of the partisan bias there."

Not surprisingly, each side blames the other for letting things get so bad.

To hear Republicans tell it, Obama has eroded their trust by going around Congress time and again with executive actions, particularly on health care and immigration, where he took steps as far back as 2012 to extend deportation stays and work permits to hundreds of thousands of younger immigrants in this country illegally.

Instead of easing the strategy after Democrats took a beating in the November midterm elections, Obama doubled down with a raft of new immigration directives affecting millions more immigrants.