For decades, the question of whether and how Palestinians might build a state in their homeland has been at the center of Middle East politics — not only for the Palestinians, but for Arabs around the region, many of whom regard the Palestinian cause almost as their own.

Forcing Palestinians out of their remaining territory, Arabs say, would doom Palestinian statehood and destabilize the entire region in the process.

So it was a nightmare for the Palestinians’ closest Arab neighbors, Egypt and Jordan — and a dream for Israel’s far-right-dominated government — when President Trump proposed moving everyone out of the Gaza Strip and onto their soil, an idea he repeated in a White House news conference with Prime Minister Benjamin Netanyahu of Israel on Tuesday.

Egypt and Jordan have responded with categorical “nos” — even if their reasons aren’t all borne out of pure concern for the Palestinian plight: Cairo dreads what Palestinian refugees in Sinai would mean for Egypt’s security. Militants could launch attacks at Israel from Egyptian soil, inviting Israeli retaliation, or be recruited into the local insurgency in Sinai that Egypt has battled for years. Jordan’s king has to reckon with a population that is more than half Palestinian, so to accept more such refugees could further raise tensions.

That refusal has been backed up by political independents and opposition figures in Egypt, along with mouthpieces for the country’s authoritarian government, underscoring how the Palestinian issue can unify even the bitterest political opponents there.

Khaled el-Balshy, the editor of one of the few remaining Egyptian media outlets that are not pro-government and the head of the national journalists’ union, issued a statement on Wednesday calling Mr. Trump’s proposal “a clear violation of human rights and international laws.”