So then support your opinion... I personally don't agree with the Nixon example, but that hardly discredits the overall point of the article (which that excerpt doesn't even encompass).
The Nixon example was particularly stupid, for starters. I don't even know anything about John Ashbrook in 1972, but I know that attempting to use a failed primary challenge against a sitting President running for re-election in 41 years ago as a portent of why it would be politically unwise for a PAC to support a more progressive candidate against the frontrunner for an election 3 years in the future makes no sense at all. A total non-sequitur.
The article chides "the professional left," coined by Robert Gibbs.

The author David Weigel is a registered Republican who voted for Gary Johnson in 2012 and John Huntsman in the primary btw, if case you didn't know. You're citing a Republican to basically tell us why he thinks the left wing of the Democratic party sucks.
He knocks the PCCC for throwing early support behind Warren. Gee, god forbid a progressive PAC actually support the progressive candidate. The fukk is the point of their existence if they're just supposed to shut up and rubber stamp whatever corporatist establishment Democrat happens to be the presidential frontrunner?
He says the left should be creating a grassroots political infrastructure instead of using a presidential campaign as a testing ground and calls it "risky."

They already are a grassroots operation. The PCCC has been heavily involved in congressional races at the state and federal level, they push hard for progressive policy issues, they successfully helped in getting businesses to sever ties with ALEC and they basically drafted and funded Elizabeth Warren into her Senate seat and raised almost a million dollars for her. But they're supposed to not even verbally support Warren, the candidate who better represents their vision, and who's political career they practically made because it's "risky"? What does that even mean? Pressure from the left on Hillary or whatever establishment Democrat is good thing. You can do the undercover grassroots establishment, which the PAC in question already does, and still support a progressive presidential candidate. If Warren does run against Hillary and she loses, that would still be a better thing for progressive policy than if liberals all just shut up and worked in the shadows. Hillary would be forced to respect the pressure from the left to garner base support from the left and AT LEAST speak the progressive language on policies, which would help get them in the public debate. The notion that a Warren run would make the left more irrelevant is baseless.
The Grover Norquist comparison is silly too because nothing about Grover Norquist is grassroots. Grover Norquist came from the Chamber of Commerce and is as well-connected to and well-funded by corporate America a political operative as anyone. And Grover Norquist works on behalf of corporate America, who funds the Republicans. He's basically just an effective middleman between Microsoft, Pfizer, ExxonMobil, etc. and the GOP. The notion that there's anything equivalent to him on the left that could push for progressive policies as effectively is ludicrous.
The article basically just boiled down to "progressives are annoying and should shut up." Shocking to hear from a Republican giving Democrats supposed well-intentioned advice (yeah, sure).