WASHINGTON — Hillary Clinton is facing one of the most fateful decisions of the presidential primary season: what to say about the Trans-Pacific Partnership and when to say it.

Politically, Clinton’s choice is a true Sophie’s Choice. Whatever she does is guaranteed to generate criticism and anger key constituencies. The best case scenario for Clinton would have been to have negotiations blow up and therefore avoid having to take a position.

Now that failure is not an option, there are four possible permutations. Clinton could embrace the agreement swiftly, or she could embrace it eventually. Conversely, she could come out against it quickly, or come around to opposing it eventually.

The first option — quick support — is my preferred one, substantively and even politically. The final option — eventual opposition — is the one that is both the most likely and, I would argue, the worst for her politically.

Indeed, timing is as important as the substance. Whichever way Clinton goes on the trade deal, this would be a good moment to channel her inner Macbeth: “when ’tis done, then ’twere well it were done quickly.” Waiting would simply reinforce the perception of Clinton as a poll-tested, finger-in-the-wind politician with few core convictions other than that she should be president.

Here’s my less-than-optimistic case for quick support. It’s not only the right outcome — it’s the one that I think Clinton believes in her heart is the right outcome. Yes, she has issues with the base — or, more accurately, the base has issues with her. Labor would go berserk if she were to support the deal. At this point, with Vermont Sen. Bernie Sanders leading in the New Hampshire polls — and already having announced against the TPP — Clinton is not exactly operating from the position of dominance she once envisioned.

And yet, Clinton has already taken steps to shore up her bona fides with the base. For environmentalists unhappy about the TPP, she came out, disappointingly in my view, against the Keystone XL pipeline. For labor, she came out, even more disappointingly, against the Cadillac tax on overly generous health care plans.

At some point, has Clinton not earned enough leeway and goodwill to break with the base? Supporting the TPP would cost her, certainly — but it would also demonstrate her independence and authenticity. “This TPP sets the gold standard in trade agreements to open, free, transparent, fair trade,” Clinton declared in November 2012.

Sure, Clinton can argue that what was agreed to fell short of the glittering standard she once endorsed. But there is something to be said for not looking like a craven, desperate flip-flopper. Isn’t there?

But Clinton’s instinct, her book title notwithstanding, is to evade hard choices. She poked fun at this tendency on “Saturday Night Live,” referring to her dithering on the Keystone pipeline and same-sex marriage. “Nothing wrong with taking your time,” Clinton as Val the bartender, told Kate McKinnon as Hillary. “What’s important is getting it right.” You have to wonder whether the impending trade deal was far from her mind.

And Clinton’s campaign performance to date on trade has been less than encouraging. She wouldn’t take a position on whether Congress should approve fast-track negotiation authority for the president because that was an obscure, inside-the-Beltway legislative issue, her campaign fatuously insisted.

Until, that is, she came out against it — kinda sorta, first saying that Obama should use the moment to negotiate a better deal, then adding that she would “probably” vote against it. At least if it failed to include aid for displaced workers, which did eventually pass. In other words, clear as mud.

So will Clinton, facing Sanders in the Democratic debate Oct. 13, take a decisive position for or against the deal? Or will she duck, citing the need to study the as-yet-unreleased text (it won’t be public for more than a month) before reaching a final conclusion?

History suggests she will duck. And also that, after much hemming and hawing on Clinton’s part; after pundits like me pouncing on her evasiveness and inconsistency; after incessant pounding from interest groups and activists, Clinton will end up opposing the deal.

That would be the worst of all worlds — which is, all too often, exactly where Clinton finds herself.

Ruth Marcus’ email address is ruthmarcus@washpost.com.