Could 2016 be the election in which the alliance between labor unions and the Democrats is ruptured? That might have sounded improbable — until the latest news from the International Brotherhood of Teamsters, which just decided to withhold an endorsement of Hillary Clinton.

The Teamsters’ 26-member board, meeting this week in South Carolina, voted unanimously against endorsing a candidate just yet, according to James Rosen of Fox News. He called it “an intentional snub to Democratic front-runner Hillary Clinton.”

Truckers are hopping mad at Mrs. Clinton for coming out against the Keystone oil pipeline. They also have their eye on Vice President Biden, a labor lackey who is wavering over whether to jump into the race. That may well be their main motive.

Mr. Rosen reports, though, that another thing is holding them back. The drivers want the chance to sit down with the Republican candidates — “most notably,” according to Mr. Rosen, Donald Trump. The Donald has taken a more protectionist line than other candidates in the GOP.

It would be unwise to make too much of this report. Protectionism is a poor policy, and many are the scribes who’ve grown old and gray waiting for Big Labor to come to recognize the job-killing nature of the socialism of the Democratic Party.

In recent years, moreover, only one Republican has pointedly reached out to labor. That was Sarah Palin, whose husband was a member of the Steelworkers. Sarah herself, from her days with the telephone company, was a card-carrying sister of the International Brotherhood of Electrical Workers.

Palin’s outreach to working Americans drove Richard Trumka of the AFL-CIO to fly to Alaska to give a speech attacking the ex-governor. Palin responded with a widely read Facebook posting headlined “Union Brothers and Sisters, Join Our Commonsense Cause.”

The Alaskan, for other reasons, failed to prosper politically, and none of the GOP gedoylim has picked up the theme. That strikes me as a tragedy, particularly given the bitterness of the jobs recession that has swallowed the Obama presidency.

The Teamsters, with more than 1.3 million members, is the fourth-largest union in America. The three larger unions represent primarily teachers and state and local government employees and many service employees working for the government.

So I like to think of the Teamsters as the largest union invested mainly in the real economy. It knows that the average annual unemployment rate during the first six years of the Obama presidency is something like a catastrophic 8.25 percent.

Even as millions quit looking for work and even as the Federal Reserve has ballooned its balance sheet by more trillions of dollars in an effort to generate jobs, unemployment is still above its average in the generation following World War II.

That’s when the Teamsters won their greatest triumph, the Master Freight Agreement, and ushered over-the-road truck drivers into the middle class. In 1972, they helped re-elect President Nixon, who’d just sprung from jail the union’s most famous president, Jimmy Hoffa.

Could the Teamsters swing behind Donald Trump? It’s hard to see Mr. Trump’s penchant for protectionism as a plus for truck drivers. It’s always been a job-killer in the long run. Yet the Teamsters are dug in against the administration’s Trans-Pacific Partnership.

Then, again, that has been supported by Mr. Biden. So the Teamsters may not have an anti-free-trade option. The biggest jobs program would be a cut in capital-gains and corporate taxes and tax reforms to enable American corporations to bring back the couple of trillion dollars they have parked overseas.

That’s not something the Teamsters (or anyone else) are going to find among the Democrats. Yet Mr. Trump hadn’t even published the details of his tax plan when the Teamsters Web site was linking to articles attacking it as a “giveaway to the wealthiest.”

In the end, the prospect that a leading union will swing behind a Republican is probably going to prove to be a vain hope. But surprises are always possible, as labor learned when the Soviet Union was collapsing and members of the Polish union Solidarity were mounting their revolt against Communism.

Who was the leader who came to visit them? Whom did they cheer from their cranes and scaffolds? It was Prime Minister Margaret Thatcher, the most free-market leader in Europe.

Is there a Republican who can make that kind of connection with labor? If so, it won’t be just the Teamsters who are looking around.

This column was first published in the New York Post.