President Trump’s greatest hurdle in persuading Americans that there is a national security crisis on the southwest border may well be his own credibility.

Mr. Trump is using a rare currency — a prime-time address to the nation — to make the case for a border wall with Mexico, an issue that has forced a partial shutdown of the federal government.

His penchant for superlatives — “the best,” “the worst,” “never,” “always,” and now, “crisis” — and his record of falsehoods, misstatements and exaggerations on the topic will likely be challenged as never before .