Too many pundits to list are offering Senators Rubio and Cruz the unsolicited advice that they should concentrate their fire on Trump in tonight’s debate, rather than on each other. One reason the senators may not take the advice is that they saw how Chris Christie’s attack on Rubio worked out: It hurt Rubio, but didn’t help Christie. Rubio may well figure that going after Trump will help Cruz, not him, and vice-versa. But given the state of the race, it seems like a risk worth taking. Taking on Trump may not get either senator to the nomination, but it’s hard to see any other route to it that stands a chance of working.

Rubio may have another reason to stand up to Trump tonight. I suspect that one of the things holding him back is a lingering doubt on the part of Republicans about his toughness. (His boyish looks don’t help him on this score.) Taking Trump on would dispel that doubt–if Rubio does it successfully. Trump fans on social media sometimes deflect criticisms of their candidate by saying “but he fights!” Up against the frontrunner, will Rubio?

(disclosure)

Update: And while I’m giving free advice based on hunches, I’ll add this: I suspect that the most effective line of attack on Trump is not that he isn’t a conservative, or that he has taken unconservative positions in the past. It’s that he isn’t on the side of the little guy, but has tried to make a lot of money over the years by screwing the little guy; that he constantly lies; that he flips back and forth between positions over the course of a day; that, in short, you can’t trust him.