Daniel Henninger explains for Wall Street Journal readers why he believes the Obamacare rollout debacle represents “the tip of the iceberg for the magical Obama presidency.”

All of a sudden, from Washington to Riyadh, Barack Obama’s credibility is melting.

Amid the predictable collapse the past week of HealthCare.gov’s too-complex technology, not enough notice was given to Sen. Marco Rubio’s statement that the chances for success on immigration reform are about dead. Why? Because, said Sen. Rubio, there is “a lack of trust” in the president’s commitments.

“This notion that they’re going to get in a room and negotiate a deal with the president on immigration,” Sen. Rubio said Sunday on Fox News, “is much more difficult to do” after the shutdown negotiations of the past three weeks.

Sen. Rubio said he and other reform participants, such as Idaho’s Rep. Raul Labrador, are afraid that if they cut an immigration deal with the White House—say, offering a path to citizenship in return for strong enforcement of any new law—Mr. Obama will desert them by reneging on the enforcement.

When belief in the average politician’s word diminishes, the political world marks him down and moves away. With the president of the United States, especially one in his second term, the costs of the credibility markdown become immeasurably greater. Ask the Saudis.

Last weekend the diplomatic world was agog at the refusal of Saudi Arabia’s King Abdullah to accept a seat on the U.N. Security Council. Global disbelief gave way fast to clear understanding: The Saudis have decided that the United States is no longer a reliable partner in Middle Eastern affairs. …

… What is at issue here is not some sacred moral value, such as “In God We Trust.” Domestic politics or the affairs of nations are not an avocation for angels. But the coin of this imperfect realm is credibility. Sydney Greenstreet’s Kasper Gutman explained the terms of trade in “The Maltese Falcon”: “I must tell you what I know, but you won’t tell me what you know. That is hardly equitable, sir. I don’t think we can do business along those lines.”

Bluntly, Mr. Obama’s partners are concluding that they cannot do business with him. They don’t trust him. Whether it’s the Saudis, the Syrian rebels, the French, the Iraqis, the unpivoted Asians or the congressional Republicans, they’ve all had their fill of coming up on the short end with so mercurial a U.S. president. And when that happens, the world’s important business doesn’t get done. It sits in a dangerous and volatile vacuum.