Don’t believe the New York Post columnist and Commentary editor? Perhaps you’ll believe history.

Unless something extraordinarily dramatic comes along to change the course of the US economy and the sentiments of the American people in the next six months, Barack Obama is finished.

That conclusion is inescapable from the history of US presidential politics since 1945.

Obama is now below 40 percent in job approval in the Gallup poll. Yes, as the political scientist Larry Sabato points out, almost every president since FDR has fallen to that level. But of those seven presidents, five (Harry Truman, Lyndon Johnson, Gerald Ford, Jimmy Carter and George H.W. Bush) went on either to lose the next election or to not run again.

The Truman story is complex: He was at 35 percent in 1946 yet won in ’48 — but he fell back into the 30s in 1951 and opted against running for a second full term.

The exceptions are Ronald Reagan and Bill Clinton. But the essentials of those wins should terrify Obama’s supporters.

Reagan hit 35 percent in 1983. By year’s end, though, the economy had lifted out of a dreadful recession to growth at an annual rate of 4.5 percent. In ’84, it grew at a sizzling 7.2 percent. The unemployment rate fell from 10.2 percent in 1983 to 7.2 percent on Election Day 1984 — and Reagan won 49 states.

Clinton hit 39 percent in September 1994, and was slammed with a GOP triumph in the midterm elections two months later. But by the end of ’94, the economy had grown 4.1 percent. Growth dropped to 2.5 percent in ’95, but bounced back to a healthy 3.7 percent in ’96. Plus, unemployment on Clinton’s watch fell from 7.5 percent in 1992 to 5.4 percent in ’96.

So how do things look for Obama? Bad — in every particular.