President Barack Obama said Monday that the time has come for Iran to decide whether to accept a nuclear deal, and he acknowledged "very real differences" with Israeli Prime Minister Benjamin Netanyahu over the negotiations. 

In a campaign speech, Netanyahu criticized the international negotiations with Iran and portrayed his scheduled address to the U.S. Congress next month as an attempt to describe Israel's objections. 

"A bad deal with Iran is forming in Munich that will endanger Israel's existence," Netanyahu said, referring to talks over the weekend in Germany between U.S. Secretary of State John Kerry and his Iranian counterpart. "Therefore I am determined to go to Washington and present Israel's position before the members of Congress and the American people." 

Obama said the world powers negotiating with Iran are unified in their offer, while acknowledging the forceful opposition from Netanyahu. 

"I don't want to be coy -- the prime minister and I have a very real difference around Iran's sanctions," Obama said at a White House news conference with Chancellor Angela Merkel of Germany, another party to the talks. 

The president defended his decision not to meet with Netanyahu while he's in town at the invitation of Republican House Speaker John Boehner. Obama cited the precedence for U.S. presidents not to meet with foreign leaders who are facing an upcoming election, to avoid the perception of trying to influence the outcome. 

"As much as I love Angela, if she was two weeks away from an election, she probably would not have received an invitation to the White House, and I suspect she wouldn't have asked for one," Obama said to a smile from Merkel, although she did not weigh in with her own perspective. 

Democratic lawmakers fear Netanyahu will use the speech to lawmakers to try to embarrass Obama and further his own re-election prospects. A handful of Democratic House members have said they will skip it, and liberal Independent Vermont Sen. Bernie Sanders on Monday became the first senator to announce he won't attend. 

"The president of the United States heads up our foreign policy and the idea that the president wasn't even consulted -- that is wrong," Sanders said in a statement. "I am not going. I may watch it on TV, but I'm not going." 

Israel has long claimed a nuclear-armed Iran would pose a grave threat to world peace and security. Israel views a nuclear-armed Iran a threat to its very existence, noting calls by Iranian leaders for destruction of the Jewish state. Iran says its nuclear program is for peaceful purposes, but critics in the West dismiss such explanations. 

Obama warned Tehran that he sees no reason to further extend negotiations for a basic agreement next month. 

"We're at a point where they need to make a decision," Obama said. He said the only question remaining is, "Does Iran have the political will and desire to get a deal done?" 

Iranian officials on Sunday signaled a willingness to come to an agreement. "This is the opportunity to do it, and we need to seize this opportunity," Iranian Foreign Minister Mohammed Javad Zarif told a gathering of the world's top diplomats and defense officials. He said an extension would be in no one's interests. 

Obama argued the extended negotiations with Iran have been time "well spent." 

"The program has not only been frozen, but with respect to, for example, 20 percent enriched uranium, they've reversed it, and so we're in a better position than we were before the interim program was set up," Obama said. 

Boehner's office objected to that assessment. "President Obama tried to clean up his debunked claims on Iran's nuclear program today but he still couldn't help but repeat a false claim. Now more than ever, it's critical that the American people hear the truth about the threat posed by Iran's nuclear program," said Boehner spokesman Cory Fritz.