Donald Trump’s dominance of polls nationally and in early states has fostered nervousness among Republicans eager to elect a reliable conservative next November. His extremist proposals (most recently, halting visas for Muslims), perpetual vagueness (“You don’t want to hear how I’d handle [terrorism] … we are going to handle it so tough!”), and confusion about our system of government (his “executive order” mandating death for cop killers) raise bright red flags about a man whose nomination could torpedo the entire party.

Meanwhile, Marco Rubio, the most electable conservative, has struggled to gain sufficient traction as the media obsesses over Trump and his outrageous statements. But Rubio can become a national hero at tonight’s debate with a gambit long avoided by the other candidates: he can declare he will not vote Republican if Trump is the nominee.

The GOP has been terrified such a statement would provoke an independent run by a sore-loser Trump after the convention. But think about it: after months of leading in the polls, do we really expect a Trumpaholic like Donald to just fall in line if defeated by Rubio, Cruz or Christie? Realistically, his name will almost certainly be on the November ballot nationwide as an independent if not as a Republican; the golden question is whether Republicans can nominate a talented, unifying personality who can defeat both Trump and whichever person the Democrats nominate.

Rubio’s best model for reclaiming sanity and responsibility for his party lies in the 1992 presidential campaign.

That year, then-Gov. Bill Clinton showed he really was a different kind of Democrat in an appearance before Jesse Jackson’s Rainbow Coalition, blasting hip-hop singer and black activist Sister Souljah for her praise of that year’s LA riots (“why not have a week and kill white people?”).

Clinton’s choice of venue was crucial; he not only rejected black extremism, but did so in front of a crowd of hard-left African-Americans. Given Jackson’s unpopularity among moderate voters and the public perception that the civil rights leader’s agenda dictated Democratic policy, Clinton’s “Sister Souljah moment” became a touchstone for the candidate’s New Democrat approach.

At tonight’s debate, Rubio can give the real-estate developer the full Sister Souljah treatment by detailing – in Trump’s face – his unfitness for public office and by declaring that any Trump-led GOP ticket will necessitate an independent but conservative challenger, lest the country be stuck with two completely unacceptable choices.

Such a pronouncement would be the top headline emerging from the debate, demolishing the phobia timorous Republicans hold of alienating Trump and his voters. It would also help attract winnable non-Republican voters concerned that a Rubio-led GOP will tolerate Trump-style craziness. In fact, it could shake up the entire 2016 race, helping coalesce currently splintered mainstream GOP voters behind Rubio as a bold, sane conservative pick.

The urgency of this moment goes well beyond quashing Trump’s impetuosity. The current Supreme Court has only two young liberals. That means an eight-year Republican administration could leave behind a 7-2 conservative majority remaking American jurisprudence for a generation.

But who would President Trump’s Supreme Court justices be? Do we have any idea? (If asked, he’d probably say something like “I will pick great people! Just watch me.”) What’s to prevent him from, on a whim, nominating someone to please a Democratic Senate, thus squandering a vital opportunity? Trump is not a movement conservative; he is an opportunist using conservatism to serve the sole cause he cares about: his own ego. Only a true believer like Rubio can be trusted with the power to appoint judges with lifetime tenure.

A presidential race is not a reality show. Trump has enjoyed GOP respect for too long, and a Rubio-led Sister Souljah debate moment can mock the front-runner’s pretentions the way Walter Mondale’s “Where’s the Beef?” halted Gary Hart’s “new ideas” momentum in 1984 and Lloyd Bentsen’s “Senator, you’re no Jack Kennedy” destroyed Dan Quayle’s dignity forever.

Tonight may be Rubio’s last opportunity. He should grab it.

David Benkof is Senior Political Analyst at the Daily Caller. Follow him on Twitter (@DavidBenkof) or E-mail him at DavidBenkof@gmail.com.