Here’s a strategic question heading into tonight’s one and only gubernatorial debate. Who has the most to lose?

Some might say Gov. Jerry Brown, since his big lead leaves almost nowhere else to go but down. Could a big mistake that goes viral (since almost no one will be watching the debate live) make this race?

Some might say Neel Kashkari, who must make a strong impression or risk never being heard from ever again.

I’d argue that the person with the most to lose won’t be on the stage. That person? Dan Richard, chair of the High Speed Rail Authority.

Richard has done a terrific job keeping high-speed rail moving in the face of multiple legal challenges, a collapse in political support for the project, and the constant carping of much of the media (yours truly has referred to it as a “zombie”).

But that momentum faces a moment of peril tonight.

Why? Because high-speed rail is certain to come up, and it’s perhaps one issue where the differences between the candidates – both moderates who actually don’t have deep disagreements on the issues — are stark. Kashkari calls it the “Crazy Train” and wants it repealed. Brown has defended it strongly.

The trouble for high-speed rail is that this may be the one issue upon which the challenger, Kashkari, has the force of public opinion at his back. And Brown knows it. So the question is: will Brown remain stalwart in his defense of high-speed rail, despite the fact that highlighting support could carry a political cost?

You might think that the answer to that question is easy: “Yes, Brown must defend what he’s always defended, and why wouldn’t he, since he’s so far ahead at the polls.” But there are reasons to doubt how Brown will respond. For one thing, he’s famously unpredictable. For another, he is highly cautious politically, and seems to put a premium not just on winning but on winning by big margins.

We saw this propensity for caution and for running up the score when he avoided making specific commitments in his race against Whitman (leaving him without a mandate but a resounding victory), and when he avoided seeking permanent tax increases (which meant he didn’t resolve the state’s long-term fiscal problems) and instead chose to make Prop 30’s taxes temporary, giving that measure an overwhelming victory.

The cautious Brown has been very much evident in this re-election campaign. He’s not on display, and he is not seeking a mandate for his second term. If he’s pressed by Kashkari on high-speed rail, could he duck, or offer a less than full-throated defense of high-speed rail? And if he seems to be backtracking on high-speed rail, what does that mean for the project? Could an equivocal statement be taken as a signal by project opponents that Brown’s support is less than strong?

What Brown says, and doesn’t say, about high-speed rail will probably be the most significant event of the evening. If I were Dan Richard, I’d be nervous.