Eliot Cohen has an idea:

It is time for a third candidate, and probably for a third party.

Some people will dismiss this notion as absurd. However, only those prescient enough to have forecast Trump’s success have the standing to certify impossibilities. If the Trump candidacy has blown up every other aspect of political conventional wisdom, why not this one?

If anti-Trump Republicans want to split off and run their own candidate, they are free to do so. However, they should do this with the understanding that their protest will amount to very little, and they will allow Trump and his supporters to blame them for his defeat. It is more than a little amusing that the anti-Trump protest idea is being supported by otherwise reliable Republican partisans who would normally mock and deride third-party voters for wasting their votes. I won’t say that about an anti-Trump protest candidacy, but I will say that they are helping to let to Trump off the hook for what most assume will be a failed general election campaign. Trump’s die-hard Republican opponents don’t need to go to the trouble of running a third-party candidate or taking over an existing third party nomination to keep Trump from being president. They can do that simply by not voting for him. Just by withholding their support, committed anti-Trump Republicans can get what they want while letting Trump take the fall for losing in November. If they go through with a protest candidacy, they will more than likely just embarrass themselves and destroy the political career of whichever hapless person they convince to accept the role of sacrificial lamb.

Cohen falls back on an increasingly common lazy argument to make his case: if Trump has proven conventional wisdom wrong in some things, then we can apparently discount everything we think we know about presidential elections. No one actually thinks a third-party candidate has a chance of winning or even competing in a general election. The structure of our system all but guarantees that minor party candidates have no chance. Nothing about Trump’s success in the primaries changes any of that. The fact that no one thought Trump could compete for the GOP nomination doesn’t mean that an anti-Trump protest candidate has a chance to do well in the fall. One has nothing to do with the other.

He anticipates the danger that Clinton will claim a “mandate” from her election win, and argues that this is why a protest candidacy is worthwhile:

Even if a third candidacy still yielded a Clinton victory, it would be worthwhile. It would, first, deny the Clinton campaign the illusion of a mandate from American voters who would have, en masse, turned out to reject Trump. If nothing else, a strong third-candidate vote would send her a message to govern from the center, rather than in deference to her party’s increasingly powerful left wing.

This is fantastical. If Clinton defeats her major party opponent by a wide margin, which a protest anti-Trump candidacy makes more likely, she will claim a “mandate” and the protest candidacy will have made it easier for her to do so. There isn’t going to be a “strong” protest candidate from the right, who can probably count on at most 8-10% in a general election. An anti-Trump protest candidate will increase Clinton’s margin of victory. If Trump’s opponents don’t care about that or are willing to take that chance, they should proceed with their protest campaign. But they shouldn’t start out with the faulty assumption that they are helping to rein in Clinton or keep her in check. They are doing just the opposite, because they are making it easier for her to win without moving to the center at all.

Cohen continues:

A third candidate could lay the groundwork for a new political party.

I suppose he could, but what would be the purpose of that new party? To represent the bankrupt, Bush-era GOP agenda that even most Republicans are tired of supporting? To split the center-right vote for years or decades to come, and thereby virtually guarantee Democratic victories in future elections? How many people would stick with this party once Trump has been defeated? Why would they bother? It doesn’t make any sense.