Every major sub-species of conservative has a credible candidate in the GOP primary race. So why shouldn't Democrats have a similar choice? Why shouldn't the increasingly powerful and influential leader of the Democratic Party's progressive wing seek to become the leader of the party as a whole?

For Gary Wills, a Pulitzer-prizewinning progressive historian and journalist, the case against a Warren campaign is simple: Hillary Clinton's going to win anyway, and Ms Warren can do more from the sidelines. Mr Wills praises Ms Warren for becoming "a force by sticking with what she knows better than anyone—the obscenity of banks’ high profits and workers’ low wages". As her following expands, she can pressure Mrs Clinton to embrace her more resolutely left-wing agenda. This is far better, Mr Wills argues, than having Ms Warren peddle the kind of "lyrical nonsense" and "airy nothingnesses" that inevitably fuel any presidential campaign. After all, the only thing Ms Warren's candidacy might achieve would be "to hold Hillary Clinton’s feet to the fire on populist issues", and she is already doing this, writes Mr Wills.

I'm not convinced. This argument could have been levelled with equal plausibility against Barack Obama in 2007, when he was a first-term senator running against Ms Clinton. At the time Mr Obama was among the most liberal members of the Senate, a darling of progressives, unmatched in his erudite and eloquent grasp of policy detail. It's important to remember that his chances early on against Ms Clinton's awesome political machine seemed very poor indeed. No one really thought he could win. But he did.

The question, then, is whether Mr Wills would be willing to say that progressives might have been better off in 2008 had Mr Obama set aside his campaign and all his empty lyrical nonsense about "change we can believe", and devoted himself instead to pushing an effectively unchallenged Ms Clinton to the left from outside the race. It's not clear how that would have worked, and it's not clear how it is supposed to work now.

In order for Ms Clinton to actually tack left, rather spout a few popular talking points, she must genuinely fear losing the left wing of the party to a progressive challenger. But there is nothing to fear from a rival politician, no matter how popular and persuasive, who is not really a rival—who is simply shouting from the sidelines. Ms Warren's message about the depredations of runaway inequality won't have any teeth unless she puts herself in a position to take a mortally wounding bite out of Ms Clinton's support. Now, you might think that Ms Warren is no Barack Obama. But then Mr Obama was no Barack Obama when he first jumped into the race for the Democratic nomination. I, for one, believe Ms Warren has what it takes to pose a real threat to Ms Clinton. If she loses in the end, it can be assumed that Ms Clinton will have been forced to make meaningful promises and to have cut real, substantial deals with progressive constituencies in order to come out on top. And if Ms Clinton, already weakened by scandal and the public's exhaustion with dynasty, happens to lose to Ms Warren, well, wouldn't Mr Wills prefer that?

Well, maybe he wouldn't. One senses in arguments like Mr Wills's a covert assumption that only a formidable centrist, such as Ms Clinton, stands a chance against the Republican candidate in the general election. Many similarly speculate that Ms Clinton's chances will be better yet if she is not forced to endure a bruising primary. This line of thinking encourages progressives to content themselves with the impotent avuncular socialism of Bernie Sanders (the Ron Paul of the left), and the toothsome, wholly unthreatening ambition of Martin O'Malley, and otherwise simply hope that Ms Warren's earnest and articulate defence of actual progressive convictions somehow exerts a mysterious influence on Ms Clinton. This seems to me to put rather too much faith in Ms Clinton, a proven primary loser, and too little faith in the democratic viability of progressive convictions. If, as it appears, the influence of Ms Warren and her message is ascendant, now is the time for progressives like Mr Wills to demand that she seize real influence by running for the Democratic nomination. What is it that Phyllis Schlafly used to say? A choice, not an echo. That's what Democrats ought to demand.