WASHINGTON -- If not for a certain Manhattan billionaire, Bernie Sanders' surprising strength and Hillary Clinton's relative weakness would be the big political story of the year.

Democrats are fortunate that bloody insurrection is roiling the Republican Party. Clinton -- the likely Democratic nominee -- will almost surely face either Donald Trump, who is toxic to most of the electorate, or an alternative chosen at the GOP convention and seen by Trumpistas as a usurper.

Clinton would be favored to beat either Trump or his closest challenger, Ted Cruz, whose ultraconservative views would be expected to repel independent voters. But Democrats should be thankful that John Kasich, who could have broad appeal, is almost surely too moderate to win the nomination of a Republican Party dragged to the far-right fringe by its angry base.

Given the circumstances, Democrats are allowing themselves to dream of a historic sweep in which they retake the Senate and even threaten the GOP's huge majority in the House. Before counting unhatched chickens, however, it would be wise to take a cold-eyed assessment of the Democratic race thus far. This is not the way party elders would have scripted it.

By any objective measure, Clinton is far ahead. She has won in 18 states, compared to Sanders' 14. More people have voted for her than for any other candidate, including Trump -- far more than have voted for Sanders, since most of her victories have been in primaries while most of his have come in caucuses. More important, she has such a big lead in convention delegates that it is mathematically improbable, though not yet impossible, for Sanders to catch her.

That said, Sanders is doing his best. He has won five of the last six states, including a far-West sweep last week in Washington, Alaska and Hawaii. His lackluster performance with African-American voters in the South, which Clinton swept like a conquering hero, improved notably in Michigan where he won a hard-fought primary. He continues to draw big, enthusiastic crowds, and raises money so easily -- basically, just by mentioning his campaign website -- that he can afford to stay in the race all the way to the convention.

And why wouldn't he? Calls for Sanders to drop out, at this point, strike me as premature and probably counterproductive. He embodies the views and aspirations of millions of Democrats -- including many in large states that are yet to vote, such as New York, Pennsylvania and California. What purpose would be served by denying so many people the opportunity to vote for the candidate of their choice?

Most campaigns end when they run out of money. Sanders is not quite as flush as the billionaires he rails against, but pretty close.

To win the nomination, Sanders would have to defeat Clinton in Wisconsin next week, ride that momentum to beat her decisively in the Northeast states yet to vote and end with a big victory in California. In other words, he would have to pull an inside straight. As any poker player can testify, it does happen -- but don't count on it.

Even at that, the best he could hope to do is narrowly beat Clinton in the number of delegates allocated by voters in primaries and caucuses. He would still have to convince hundreds of convention "superdelegates" -- mostly elected officials who embody the Democratic establishment -- to abandon Clinton and vote for him as nominee. So really, it's more like an inside straight flush that Sanders needs.

But look at the bigger picture: It's April and Clinton has not managed to put away a 74-year-old avowed socialist who wasn't even a Democrat until he began his campaign. Why is that not worrisome?

Clinton's disapproval rating is consistently higher than her approval -- an average of 55 percent to 40 percent, according to Huffpost Pollster. (This would be seen as a crisis if Trump's disapproval were not even higher.) Substantial numbers of voters say they do not trust her.

In isolation, let's face it, she looks beatable in the fall. Fortunately for her, the Republican Party doesn't look capable of beating anybody. But I wouldn't want to bet everything on GOP dysfunction.

It's too early for Sanders to pull out, but not too early for him to begin making clear that if he does lose to Clinton, he will support her enthusiastically and without reservation. He needs to tell supporters, in no uncertain terms, that if they stay home in November a Democratic victory is far from assured.

(c) 2016, Washington Post Writers Group