This has been a bad week for Israeli Prime Minister Benjamin Netanyahu. His relationship with the United States is frosting over, and the world is inching closer to an Iranian nuclear agreement that he sees as foolhardy and dangerous. But while he may be increasingly isolated on the world stage, the Israeli leader still has substantial leverage in Washington, which he may be able to use to undermine U.S.-Iran talks or possibly kill them outright.

The trouble for Netanyahu started Wednesday, when Secretary of State John Kerry took some shots at the expansion of Israeli settlements in the West Bank -- long criticized by the United States as a threat to peace talks between Israel and the Palestinians.

"We consider now and have always considered the settlements to be illegitimate," Kerry said from Bethlehem, where he'd met with Palestinian leader Mahmoud Abbas. Kerry noted that "the Palestinians believe that the settlements are illegal" before again calling them illegitimate. His earlier meeting with Netanyahu had been much chillier; as Reuters summed up the tour, the U.S. secretary of state "appeared to slap down Netanyahu and warmly endorsed Palestinian President Mahmoud Abbas's commitment to seeking a two-state solution."

Then, on Friday, Kerry jetted off to Geneva, where Western and Iranian diplomats are attempting to hammer out a possible nuclear deal that Netanyahu vocally opposes. The Israeli leader sees it as an Iranian ploy to reduce sanctions and divide the West, all without actually following through on promises to curb nuclear progress. The Obama administration shares that skepticism but is pushing ahead, floating the possibility of sanctions relief for Iran. Netanyahu appears to be genuinely angered about the emerging agreement, warning, "Iran got the deal of the century, and the international community got a bad deal."

The question is whether Netanyahu can, or will, do anything more than fume from the sidelines. On the one hand, he may be chastened by concerns about Europe. His partnership with the United States might go through ups and downs, but Israel's relationship with Europe appears to be on a potentially much more serious downward slide. European leaders are bullish on finding a deal with Iran and are also increasingly wary of the Netanyahu government's role in the Israeli-Palestinian peace talks, seeing it as intransigent and counterproductive. They might be willing to tolerate their disagreements with Netanyahu on one of those issues, but on both?

If Netanyahu is perceived as undermining a deal with Iran or, worse, as responsible for scuttling it outright, then Europe's support for Iran sanctions could crumble, which is the opposite of what Netanyahu wants. Further, as Jeffrey Goldberg writes at Bloomberg, Netanyahu's perceived intransigence on both Israeli-Palestinian peace talks and on Iran could leave him isolated "from the rest of the world, precisely at a moment when he needs the rest of the world to help him."

Still, Israel is not powerless. Netanyahu might be able to exert real leverage over the Iranian talks at perhaps their most vulnerable point: the U.S. Congress. The greatest hurdles to any Iranian nuclear deal will probably not be in Geneva, where the diplomats broadly agree on the contours of a final accord, but in Washington and Tehran. Domestic politics in both countries are, at best, divided on the prospect of a deal and tend to default to the status quo of mistrust and hostility. The White House and Congress are already coming into conflict; just as U.S. negotiators hint at sanctions relief, the Senate is moving forward on enacting even more sanctions, undermining the U.S. proposal.

Many lawmakers, particularly but not exclusively Republicans, are beginning to rally around the idea that any sanctions relief would be dangerous and requires their opposition. It doesn't hurt that appearing tough on Iran is a politically popular position that poses few risks for lawmakers and substantial benefit. Keep in mind that according to public opinion polls, Americans hold highly negative views of Iran. In addition, lawmakers have been denouncing the Obama administration over Middle East policy for years. Congress successfully opposed the White House's plan for limited strikes against Syria, a reminder that the administration has a very tough time getting controversial Middle East policies through a legislative body that is not predisposed to helping him.

Given that "Benghazi" remains a rallying cry for Republican lawmakers more than a year after the attack on the U.S. diplomatic mission there, it's hard to imagine them green-lighting sanctions relief for Iran. Still, some in Congress are open to the prospect of a deal with Iran: 131 members signed a letter in July saying they backed diplomacy.

This is where Netanyahu could play a major role, and potentially scuttle any nuclear deal with Iran, should one emerge from Geneva. Sanctions relief will be controversial in Congress, and Republican lawmakers will try to draw as much attention to the issue as possible so as to rally public opposition. What they lack is a public face to put on their campaign. Netanyahu can provide that: He is popular in the United States and has demonstrated a flair for rallying Congress. He's also not particularly shy about criticizing the diplomatic outreaches with Tehran. If Netanyahu continues arguing against an Iranian deal, and particularly if he does so in a way that's crafted to resonate in any domestic American debate, he could make the Obama administration's task in Congress much harder.

That doesn't necessarily mean Netanyahu would go through with this, of course. Up until the last couple of weeks, his relationship with the Obama administration had been improving, after a rough couple of years. Israel needs the United States, which Netanyahu knows; his bet, in 2008, that Mitt Romney would replace Obama in the White House failed. Still, if he decides  it's worth it, he could cause Obama a lot of trouble in Congress and in the court of U.S. public opinion – perhaps shutting down any Iranian nuclear deal in the process.