The big threat to Hillary Clinton’s campaign isn’t coming from a competitor. It’s coming from an issue. And it’s coming now, long before the first primary. Will she be for or against giving President Barack Obama fast-track authority to negotiate trade deals?

Fast track gives a president the power to make trade deals that cannot be amended by Congress. All Congress can do is accept or reject the agreement. Other countries, say fast-track supporters, won’t negotiate with Washington if they know that Congress can amend any deal they agree to.

If he gets fast-track authority, Obama will be in a position to conclude negotiations for the Trans-Pacific Partnership, a mammoth free-trade deal among 12 Pacific Rim nations that represent 40 percent of the world economy. Obama embraced the deal saying, “We have the opportunity to open even more new markets to goods and services backed by three proud words: Made in America.”

But it looks like a tough slog to get Congress to approve fast track. Because Republicans control both houses? No. Republicans support the president on this. They believe in free trade.

Obama’s problem is with Democrats. They are horrified by the Trans-Pacific Partnership. They see it as a job killer, like the North American Free Trade Agreement passed in 1993 under President Bill Clinton. A powerful coalition of labor unions, progressives, environmentalists and Latino organizations opposes the new trade agreement. That’s the Democratic Party base.

See Hillary Clinton’s problem? If she supports fast track, it will inflame liberals who will hound her over it all through the primaries. It may even propel a challenge from her left. If she opposes fast track, she will enrage the Obama administration and get blamed if he suffers an embarrassing defeat. She will also tick off Wall Street and risk losing their campaign contributions.

Technically, Clinton does not have to take a position at all. She’s no longer a member of Congress. But she’ll look like a wuss if she refuses to say where she stands on an issue that is galvanizing her party.

Democrats have been divided on trade for a long time. When Congress approved the North American Free Trade Agreement in 1993, most Democrats voted against it. Republicans delivered Bill Clinton’s victory on the trade deal.

Today, Democrats are less divided on trade. They are pretty solidly opposed. Moderate Democrats, who tended to favor free trade in the past, have been disappearing in Congress because of losses to Republicans. House Speaker John Boehner (R-Ohio) says he can deliver most Republican votes for fast track but he will need 50 Democrats to vote with them. Right now, no more than 20 House Democrats are ready to support it.

If Hillary Clinton comes out for fast track, it might get just enough Democratic support to pass. If she opposes fast track, it’s probably doomed. The fate of the measure very likely rests in her hands.

Most Democrats believe she will come out against it. After all, as a senator in 2002, she voted against giving President George W. Bush fast-track authority. It’s too risky for her to antagonize her party when she just entered the 2016 race. It may even do her some good to part ways with Obama on the issue. That would make it harder for Republicans to argue that a vote for Clinton is a vote for a third term for Obama.

Clinton is running a populist campaign. “It’s fair to say as you look across the country, the deck is stacked in favor of those already at the top,” she said in Iowa last week. Supporting a trade deal would open her up to the charge that she’s a phony.

Trade is not an ideological issue. It’s a populist issue — the people versus the establishment. Ordinary Americans are suspicious of trade deals. Economists have a hard time understanding this, but most people see trade not as an economic issue but as a moral issue.

People think it’s wrong for them to benefit as consumers from lower prices for foreign-made goods if it throws Americans out of work. Will they purchase the foreign-made goods? Of course they will — as long as they’re cheaper. That’s rational economic behavior. They just don’t think they should be allowed to.

Clinton has been all over the place on trade. She supported NAFTA when her husband was president. In her 2008 campaign for the Democratic presidential nomination, she promised to renegotiate it. As senator, she supported Bush’s trade deals with South Korea, Colombia and Panama. But she opposed the Central American Free Trade Agreement in 2005.

As secretary of state, she praised the Trans-Pacific Partnership, writing in 2011, “Our hope is that a TPP agreement with high standards can serve as a benchmark for future agreements.” She says she believes in the principle of free trade but wants Washington to be a tougher negotiator. “I believe in smart trade,” Clinton says, “pro-American trade.”

But this decision doesn’t allow a nuanced position. You have to be either for it or against it. The best outcome for Clinton would be to come out against it to protect herself politically. And then hope it passes.