In our polarized politics, the Democratic Party is trending leftward — not as sharply or as rapidly as the Republican Party has turned toward the far right, but still the party is moving further left than a generation or even an election or two ago. The stunning rise of Bernie Sanders, who proclaims himself both a democratic socialist and a political revolutionary, provides powerful evidence of desire at the grassroots for substantive and perhaps radical change.

Yet many Democrats understandably wonder, as the primary season begins, how far their party can go without risking a catastrophic loss of the White House and the Congress to Republican extremists like Ted Cruz and Donald Trump. They also wonder whether Hillary Clinton, whose surname is synonymous with triangulating centrism, will inspire their base — and whether her candidacy represents any kind of change, beyond gender.

The response of the Sanders faithful is straightforward enough: Their man is not only more faithful to progressive principles but more likely to win in November, because his "revolution" will bring out millions of new voters. (Evidently the Republican Party, which attacks Clinton relentlessly while sending out messages supportive of Sanders, doesn't agree.)

Such was the argument advanced by The Nation magazine — a venerable publication of the left, celebrating its 150th anniversary — in its recent cover editorial endorsing Sanders for president. Along with many other Nation articles over the past several months, that lengthy essay leaves the unmistakable impression that Sanders is the only progressive choice for Democratic voters.

Those who doubt Sanders' electability or qualifications may still ask themselves whether a true believer can support Hillary Clinton, however. And they might well find an answer in the pages of The Nation, too.

Just a month ago, the magazine published its 2015 Progressive Honor Roll, an annual feature compiled by John Nichols — a veteran journalist and author who happens to be a highly enthusiastic Sanders supporter. But he apparently applied no political litmus test to his list of America's "most valuable" progressives, because it included several prominent supporters of Clinton. In fact, of the individuals honored by The Nation, nearly every single one is backing her.

Topping the list is Sen. Sheldon Whitehouse, D-R.I., named "most valuable Senator," who officially endorsed Clinton back in January 2014. Rep. Rosa DeLauro, D-Conn., the list's "most valuable House member," endorsed her last April. State Senator Pam Jochum, the Dubuque Democrat who presides over the Iowa State Senate - and was chosen from among thousands of local elected officials across the country as the "most valuable state legislator" - announced her support for Clinton last October.

Cecile Richards, the Planned Parenthood president named as the country's "most valuable activist," led her organization to back Clinton earlier this month (and earned a sour-grapes dismissal by Sanders as "the establishment"). Newark, New Jersey's Ras Baraka, chosen as the "most valuable mayor," hasn't officially endorsed a presidential candidate yet, but his political organization has shown every sign of backing Clinton since last summer. And "most valuable memoir" author Gloria Steinem, the revered feminist leader and thinker, is currently campaigning for Clinton in New Hampshire.

Incidentally, Health and Human Services Secretary Sylvia Mathews Burwell also makes the list as "most valuable Cabinet member." She must observe administration neutrality in the primary but — as a former top Clinton White House official — would very likely endorse the former first lady.

Whatever their reasons for supporting Clinton, these are all serious progressive leaders who don't take a presidential endorsement lightly. Agree with their choice or not, it still carries weight.

As the first wave of voting approaches, campaign rhetoric gets super-hot and sometimes angry. Partisans on both sides will inevitably utter silly, uninformed, and even offensive remarks about the opposing candidate. But at such moments it is worth emphasizing that progressives can differ honestly over which of these two leaders can represent the nation's real interests most effectively. That will be even more important to remember when the nominee finally emerges.

To find out more about Joe Conason and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.