Labor Party leader Isaac Herzog stated the obvious when he noted today that both the Israeli government and its opposition agreed on the nuclear threat from Iran. But as much as he shares Prime Minister Netanyahu’s conviction that Israel cannot tolerate an Iranian nuclear weapon, he wants no part of a joint trip to Washington with his rival. Herzog rejected the invitation from the Likud to join the prime minister when he goes to Congress in early March. But while there are good reasons for both Israelis and American supporters of the Jewish state to question the wisdom of Netanyahu’s decision to accept the invitation to address a joint session of Congress, Herzog’s unwillingness to play along with Netanyahu’s gambit demonstrates that a move that has actually worsened the chances of Congress passing more sanctions may be helping the prime minister politically at home more than it is hurting him. By forcing Herzog to declare himself ready to trust the Obama administration to do the right thing on Iran—just at a time when it appears to be making even more concessions that endanger the security of the West and Israel—Netanyahu could be ensuring his reelection next month.

With only three weeks to go before Israeli voters head to the polls, the race between Netanyahu’s Likud and the Zionist Union bloc led by Herzog is still too close to call in terms of which party will get the most Knesset seats. But the coalition math in which either party must negotiate deals with several smaller parties in order to get to a 61-seat majority and the right to govern still favors Netanyahu. In order to become the next prime minister, Herzog is going to have to finish first by a healthy margin and then put together a tenuous coalition including the religious and centrist parties but excluding the anti-Zionist Arabs.

Pulling off such a feat is possible but not likely. And the more Netanyahu is able to position himself as the sole figure standing up to American pressure on the Palestinians and fighting against appeasement of Iran, the worse Herzog’s chances look. Thus, it might have made sense to make some gesture of national unity that would have enabled him to steal at least some of Netanyahu’s thunder in Washington. But Herzog can’t do it. Why? Because the rationale underlying his candidacy is a critique of the way Netanyahu has messed up the alliance with the United States.

Herzog rightly understood that the invitation to join Netanyahu was a political stunt and that the Likud was hoping he would say no. The opposition leader isn’t wrong to view the speech as now having a lot more to do with Israeli domestic politics than an effective effort to stop an administration determined to cut a deal with Iran on any terms, even if its provisions virtually concede its status as a threshold nuclear power and will eventually allow the regime to build a weapon with impunity. But the problem for Herzog is not in diagnosing the futility of Netanyahu’s speech or the fact that it has helped President Obama pick off wavering Democrats and therefore prevent the creation of a veto-proof majority for increased sanctions on Iran. Rather, it is in being put in the position of being Obama’s man in Jerusalem just at a time when the president seems to be betraying Israel’s interests in the Iran talks rather than just engaging in another pointless spat with Netanyahu.

There’s no question that the White House will be holding its breath on March 17 and the days following the Israeli vote hoping that somehow Herzog and his ally Tzipi Livni can prevail. Herzog seems to appreciate this and is saying nothing to indicate that he will make trouble for Obama on Iran or any other issue.

But Herzog has to be worried about two things happening that would make Netanyahu’s congressional speech more than a campaign speech.

One is the very real possibility that the U.S. will cut a deal with Iran in the next couple of weeks that will give the Islamist regime the right to hold onto to its nuclear toys and give it a chance—whether by a breakout or waiting out a freeze period such as the one suggested by the U.S. this week—that will give it a nuclear weapon. If the president who is already deeply unpopular in Israel agrees to a deal that is widely seen as undermining Israeli security, Herzog will be hard-put to continue to claim that he can defend Israel’s interests more effectively than Netanyahu by warming up the relationship with Obama. At that point, he will be forced into a stance that will be a faint echo of Netanyahu’s full-throated opposition to an Iran deal and irrelevance.

But even if a deal isn’t struck before the speech or the election, Herzog still has to be concerned about the administration’s push for Iran détente becoming more overt. Indeed, the closer we get to a deal, whether or not it is signed, the steady stream of U.S. concessions to the Islamist regime makes Herzog’s position as Obama’s favorite in the elections more untenable than ever. Though Obama would like to help Herzog, the irony is that the harder he tries to achieve his main second-term foreign-policy goal—an entente with Iran—the worse Herzog’s chances may be. While Herzog is right to say that, if elected, he would, at least initially, be able to warm up relations with Obama, being cozy with someone who is getting cozy with Iran is a very uncomfortable place to be for a man who wants to be elected prime minister of Israel.