There are few things that pollute our political system more than vacuous partisanship.

Substantive partisanship — the more beneficent variety based on ideology — is less common, largely for institutional reasons. (Because the United States doesn’t have a proportional-representation system, party members have panoply of ideological persuasions, yet they’re housed in just two major parties.)

But vacuous partisanship, that intellectually enervating phenomenon, is pervasive.

It was on full display again this week, with the launching of President Obama’s re-election campaign. In an attempt to revivify the loyal troops, the nascent campaign asked citizens to let others know that they’re “in” — that they’ll be unremitting Obama supporters through next year’s presidential election.

And many dutifully signed on, despite their inability to predict whom Obama will face in next year’s general election or whether he’ll make an(other) abhorrent policy decision in the coming year.

I have an alternative for the disillusioned left: What if instead of reflexively backing Obama, we threw our vim and vigor behind Gary Johnson’s presidential bid? What if liberal Democrats in Iowa caucused for Johnson next February — not simply to scuttle the Republican field, but to mount a substantive challenge to Obama’s policies?

The former New Mexico governor, who is set to announce his candidacy later this month, is a libertarian-leaning Republican.

So, as I’ve said before, I suggest the left-libertarian alliance with a little trepidation.

I know many of my ideological compadres have a visceral disdain for libertarians. But a strong showing by Johnson — buoyed by broad left-liberal support — would be a boon for several reasons.

Issue-based alliances between libertarians and the political left are needed to confront the bipartisan consensus of militarism, corporatism, mass incarceration, an unaccountable executive, and the erosion of civil liberties. The odious amalgam has become the norm that, save for a radical change, will only become more entrenched.

That’s where Johnson comes in.

The left won’t be amenable to Johnson’s agenda on most fiscal and economic issues. (I also find his enthusiastic cheerleading for school vouchers and support for privatized prisons as New Mexico governor deplorable.)

But he supports limiting America’s military footprint abroad, legalizing marijuana, and halting civil-liberty curtailment.

Contrast that with Obama’s horrendous record on foreign policy, drug policy, and civil liberties. I don’t think you’d see Johnson trying Khalid Sheik Mohammed before a military commission or tacitly accepting the inhumane treatment of Bradley Manning.

And Johnson doesn’t have the paleoconservative streak of well-known libertarian congressman Ron Paul, making the former New Mexico governor more palatable to the left. (Paul endorsed Chuck Baldwin, the Constitution Party’s presidential candidate in 2008, saying his views are “very, very close to mine.”)

Second, it would show the Obama campaign the left won’t meekly fall in line. The typical tack would be to get behind Obama because he is the “lesser of two evils.” But that would neuter the left’s sway from the get-go.

A characteristically perceptive Glenn Greenwald made the point well earlier this week, bemoaning the “impotence of the loyal partisan voter.”

Instead of being preemptively co-opted by the Obama campaign, this approach would underscore the left’s ire in a substantive way.

Maybe then we could break out of the insipid paradigm described in Matt Bai’s book The Argument:

“It was, to them, an endless game between two teams, one blue and one red … It wasn’t that they didn’t care about the problems of contemporary American life; it was more that they seemed to believe those problems would somehow be solved by the simple act of electing more Democrats. So certain were they in this conviction … that they had completely forgotten to ask themselves why they believed it.”

Substitute “Obama” for “more Democrats,” and you’ve hit the problem.

Contrary to the claims of third-party enthusiasts, there is a difference between Democrats and Republicans. But there’s little reason to think that any president, absent a seismic shift, will deviate from the bipartisan baseline: Corporatist advisers and economic policies. Wide claims of executive power. Interventionism abroad.

These won’t be overcome by the tribalistic tactics of partisans. We need a tectonic change catalyzed by Iowa caucus-goers — not a reinforcement of the status quo.