Here is a scenario that should keep Democrats awake at night.

Bernie Sanders keeps doing well in the big Northern and Midwestern states. On Tuesday, he wins Ohio, maybe Illinois and Missouri, which sets him up to be competitive in the big, late primaries, notably New York and California.

Those wins may not be quite enough to deny Hillary Clinton the nomination, but the momentum is clearly with Sanders. The contest goes all the way to the convention. If Sanders is denied nomination, there will be a lot of deflated and disappointed Democrats.

Clinton supporters may hope they will turn out in force in November, if only to stop Donald Trump. But there has to be more to it than that. A lot of the independents who went for Sanders could stay home, or even switch to Trump. Somehow, she needs to discover her inner progressive.

Meanwhile, on the Republican side, the stakes this Tuesday are equally high. If Trump wins both Florida and Ohio, he is very likely the nominee. If Governor John Kasich can beat him in his home state of Ohio, the race drags on a while longer; and maybe Trump's very real liabilities finally start catching up with him.

But the likelihood is that Trump will continue to do well in the very states where Hillary does badly. And that's why a Trump-Clinton race would be so ominous for Democrats.

Ted Cruz, speaking at the unusually mannered Republican debate last Thursday argued that Trump would be the easiest for Clinton to beat. But Cruz, Trump's closest rival, was whistling Dixie. Cruz doesn't travel well, and Trump does.

In fact, Karl Rove, in a Wall Street Journal op-ed piece trying to argue Trump's weakness against Clinton ended up demonstrating Trump's strength.

Trump polled worse against Clinton in much of the country, Rove pointed out, except for a few states ... Like, uh, Florida, Ohio, Pennsylvania, where he out-polls her. Oh, only those -- the very toss-up states that will decide the election!

So let us count the possible scenarios:

One is that Sanders keeps gaining on Clinton, but falls just short. A rather battered Hillary Clinton is the nominee. At that point, there is not much for Democrats to do except to keep pointing out all that is crazy about Trump, and to try to find a running mate who will bring along some of the Sanders energy. Sanders himself? Elizabeth Warren (which would give a senate seat to the Republicans)? It's not a long list.

Trump would know just how to go after Clinton's weaknesses. Jim Sleeper, writing in Salon, quotes Nathan J. Robinson, imagining how Trump would savage Clinton in the general election:

Everything she says is a lie.... She made up a story about how she was ducking sniper fire! There was no sniper fire. She made it up! How do you forget a thing like that? She said she was named after Sir Edmund Hillary, the guy who climbed Mount Everest. He hadn't even climbed it when she was born! Total lie! She lied about the emails, of course, as we all know, and is probably going to be indicted. You know she said there were weapons of mass destruction in Iraq! It was a lie! Thousands of American soldiers are dead because of her. Not only does she lie, her lies kill people. ....Check the polls, nobody trusts her. Yuge liar.

Now, Trump happens to be the bigger phony by a country mile, but how do you campaign against that without degenerating into the same kind of playground food-fight that Trump used to savage the rest of the Republican field?

I recently got a fundraising appeal from an old friend who is an enthusiastic Clinton supporter. His letter said in part, speaking of Sanders:

"So, he is not going to be the nominee; he will not be President; and, if the skies opened and he was elected he would be surrounded by an impossible Congress."

This is the standard Clinton line -- she is inevitable, so get with the program.

Sorry, pal, this doesn't do it. A front-runner who keeps under-performing expectations cannot run and win on her presumed inevitability. She has to make a better case than that.

Alternatively, suppose Sanders runs the table in the remaining primaries, persuades the super-delegates that he has the momentum against Clinton, and would be the stronger nominee in November. This really is a revolution.

Would centrist Democrats, worried about their own seats, support Sanders? Would the surge of enthusiasm on the part of the young make up for losses elsewhere? How would Sanders fare when subjected to serious red-baiting? Would Michael Bloomberg, as a business-oriented Democrat decide to run after all? A Sanders nomination is not all sunshine, either.

On the Republican side, the stakes are just as high. The recent entente among all the non-Trump candidates suggests that if they can keep Trump from getting an absolute delegate majority before the convention, he just might be stopped. Kasich could be the one with some momentum, now that most of the Southern primaries are behind us.

Can we imagine, say, a Kasich-Cruz ticket? That would be tougher for Democrats to beat than the opposite.

For all of the populist ferment on both sides, the general election could end up being an establishment Democrat (Clinton) against an establishment Republican (Kasich), and then where would the populist rage go? And would Trump run as an independent on the grounds that the GOP convention did not respect the fact that he came into the convention with the most delegates? In a three-way, would he carry a few states, possibly throwing the election into the House and President Paul Ryan?

For more than three decades, there has been a great deal of downward mobility in America, even as life has become sweeter and sweeter for the elite. As recently as 2008, there was no place for it to go in our political system. For many white working class male voters, the disaffection was compounded by partial gains of demographic groups whose aspirations had been even more suppressed -- blacks, women, people with disabilities, "despised minorities" in Louis Brandeis' phrase, such as gays and lesbians, and the symbolism of an African-American president.

By 2010, some of that rage had found a home in the Tea Party movement, much to the dismay of silk stocking Republicans. Today the pocketbook frustrations can either go right, with Trump, or left, with Sanders. They are not going away,

For progressives, the true nightmare scenario is that Hillary Clinton narrowly wins the nomination but the momentum keeps getting away from her; and on the Republican side, as much as the party elite loathes Donald Trump, a lot of Republicans hold their noses and rally behind him.

He keeps talking populism, but at the end of the day he's a businessman. He will say what he needs to say to rally the disaffection, but do we really think he will increase taxes or regulation on big business? Let him scapegoat foreigners, or Muslims or African-Americans, as long as he keeps the anger away from Wall Street.

Is Trump that much of a cynic? Is the Republican elite? Are you kidding?

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Robert Kuttner is co-editor of The American Prospect and professor at Brandeis University's Heller School. His latest book is Debtors' Prison: The Politics of Austerity Versus Possibility.

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