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A federal judge issued a temporary restraining order on Friday that temporarily blocks President Donald Trump's travel restrictions, but larger questions remain about whether the executive order is legal and constitutional.

As of now, the answer remains unclear, because no court has issued a final judgment on the executive order's merits. There are at least eight major lawsuits opposed to the order, and in seven cases, judges have begun ruling on parts of the order.

One is closed.

But the legal challenge that found the most daylight is in Washington state, where the state's attorney general filed suit against the Trump administration alleging that the order is illegal and unconstitutional.

U.S. District Judge James L. Robart ruled that the suit is "likely to succeed on the merits," which is part of the reason he blocked the order.

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The Trump administration asked an appeals court to reinstate the order immediately while the Washington case continues — an emergency request that was denied, for now, by the 9th U.S. Circuit Court of Appeals in San Francisco. That means the restrictions remain blocked unless challengers lose later arguments in the case or if the Supreme Court intervenes.

More broadly, there are three key arguments challengers are making in several of the cases.

'The order violates federal laws for implementing policy'

This argument takes aim at how the Trump administration fulfilled the order. There were wide reports of arbitrary implementation of the travel restrictions, reversals in policy on green card holders and other actions a court might find "arbitrary."

This attack, however, does not challenge the order's legality or constitutionality. A court could strike down how the administration implemented the order but leave room for the administration to try a similar policy again.

'The order violates existing federal law on immigration'

Challengers also argue that Congress already barred discrimination on national origin with a federal law more than 50 years ago.

The Immigration and Nationality Act of 1965 ended the National Origins Formula, established in 1924, which used nationality to favor Western European immigrants over Eastern European, Asian and African immigrants. Saying Congress banned discrimination based on national origin, challengers argue that the new regulations violate existing law. Defenders of the order say the president has authority under an older version of the law to suspend entrance by immigrants he deems detrimental to U.S. interests.

If this argument wins, the restrictions would be completely halted. Congress, however, could still amend the law to allow them.

'The order violates constitutional rules on due process and freedom of religion'

This argument brings up to three constitutional amendments into play — the First, the Fifth and the 14th.

Challengers argue that the executive order amounts to a "religious test" against some Muslims. If they can convince a judge that the order is hiding a religious agenda, they could win even in an area in which the president has wide powers under precedent. This argument is rooted in the Constitution's guarantee of religious freedom and equal protection under law.

Challengers also argue the order violates the right to due process, which historically applied to at least some of the categories of people affected by the order (if not all potential immigrants).

If the challengers are able to prove that the executive order violates any of these amendments, then it is totally dead — it could not be revived by the administration or Congress, because a court would find it prevented by the Constitution.