Israel's Arab parties are, for ideological reasons, extremely unlikely to join any Israeli government. But the preliminary numbers from Tuesday's election suggest they could play another crucial role in shaping the outcome: blocking Israeli Prime Minister Benjamin Netanyahu from forming another government.

"We'll prevent [Netanyahu] from forming a right-wing government," Ayman Odeh, the head of the Joint List of Arab parties, said after the exit polls were released on Tuesday night.

It makes sense that he would want to: a right-wing coalition led by Netanyahu could very plausibly pass legislation, like a bill defining Israel as the nation-state of the Jewish people, that would further marginalize Israel's Arab residents. It also would be extremely hostile to any concessions to the Palestinians in the name of peace. West Bank settlements grew rapidly under the last Netanyahu government, and the prime minister said just yesterday that there would be no Palestinian state if he were re-elected.

Odeh might actually be able to make good on his threat. The Israeli system could allow the Joint List to block Netanyahu from taking power without actually joining an alternative coalition. To understand how, you have to understand a specific quirk in Israel's parliamentary system.

(Johnny Harris/Vox)

Right now, no party can legally form a coalition — yet. To do that, the party needs to be endorsed by Israel's President, Reuven Rivlin, as the one most likely to be able to form a coalition. After endorsement, that party will have at least 28 days, and up to 42, to try to put a coalition together. During that time, no other party is allowed to put together its own competing government-in-waiting.

Whoever Rivlin chooses wins the right to take the first stab at forming a government. The president will likely decide based on a vote by the new members of parliament. Members of the Knesset can vote on whom they want to be prime minister: Likud MKs will recommend Netanyahu, while Zionist Union MKs will recommend the head of their party, Isaac Herzog. The MKs from smaller parties will also choose based on whom they'd prefer to become prime minister.

Here's where the Joint List comes in. The exit polls suggest it will have 13 MKs — making it the third-largest bloc in the Knesset.

The Joint List could recommend Herzog to Rivlin during the pre-coalition vote. If you add the Arab votes together with the center-left Zionist Union, staunchly left Meretz, centrist Yesh Atid, and center-right Kulanu, they have enough to votes (about 67 in total) to throw the presidential recommendation to Herzog, blocking Netanyahu from forming a coalition.

This might seem like a maneuver for buying time, but it could be really helpful for Herzog and the Zionist Union. If he has the right to form a coalition first, Herzog would be in a better position to woo the parties he needs into an actual coalition. That means Kulanu — whose economic approach means its support could be won by the left — and the ultra-Orthodox parties, who are a tougher sell but still could join with the right mix of socioeconomic concessions.

Having the right to form the first government would put Herzog in a more credible position to offer a package of incentives, like control over important ministries and support for their favored policies, that could bring these swing parties aboard.

The Arab parties may yet have a pivotal role to play in Israel's ongoing election drama.