I am not sure Dionne is helping. There are myths that support the identity of one who sees themselves in the center between extremes. They do not invoke any of the three he cites. Let's go through them and consider their merits.

First, that political polarization is primarily a product of how elites behave and not the result of real divisions in our country.

If this election cycle has shown anybody anything, it is how sharply you may differ from your neighbors and family about what should happen and what is most important for our body politic to embrace. I have no idea what is being proposed by suggesting something else is driving the conversation.

Second, that a vast group of party-loathing independents can be mobilized by anti-partisan messages.

What does that even mean? If people select something without regard to affiliation, that means they aren't available for mobilization. I can guess what is meant by suggesting that this is a myth but why bother? Aren't myths supposed to be more compelling than this idea? Where "independents" stand is the hockey puck in a game nobody is willing to bet upon.

Third, that Republicans and Democrats are becoming increasingly and equally extreme, so they should be scolded equally.

Okay. This observation has merit as a rebuke of the both sides do it meme. But is it a myth? Scolding? You scold someone you have some measure of power over. Who thinks they have that power? Nobody I know.

Matthew Yglesias has put out an interesting approach: that they have become the real swamp, the epitomy of the swamp. 

Trump’s administration can’t clean house because its leader is too soaked in scandal

It’s party time for grifters in today’s GOP.

@ Vox.com, July

[....] And this is the really striking thing about the current state of Republican Party politics — not the handful of crooks and spouse abusers who’ve been forced out of their jobs, but the petulant and foot-dragging manner in which they’ve been cashiered, the continued tolerance for so many apparent malefactors, and the evident lack of desire to even attempt anything resembling a proper house-cleaning.

The situation is an embarrassment to dignified political and policy professionals who happen to believe in low taxes and restricting the legal availability of abortion. And it’s obviously a drag on the Republican Party’s approval ratings, which remain dismal for a party presiding over peace and prosperity.

But nothing can realistically be done about it because there’s no way to hold your team to any kind of normal standard of conduct when Donald Trump is captain of the ship. According to his accusers, Trump is personally guilty of all these misdeeds and more. He simply can’t do anything about it even if he wanted to [....]

To me, it follows that fresh faces that have been genuinely motivated by what's going on will appeal to voters, whether liberal or moderate. Women are a change implying that they will be different, whether that's healthy to think that, we'll see.

I think these results from the July 2 Quinnipiac poll fit here: 

Voters say 71 - 20 percent, including 51 - 36 percent among Republicans, they would like to see the U.S. Congress be a check on the president.

and

American voters disapprove 78 - 15 percent of the way Congress is handling its job

and

If the election for the U.S. House of Representatives were held today, 50 percent of American voters say they would vote for the Democratic candidate, as 41 percent say they would vote for the Republican candidate. 
Independent voters back the Democratic candidate 49 - 35 percent.  There are wide gender and racial gaps:

• Men go Republican 50 - 42 percent as women go Democratic 58 - 33 percent;
• White voters are divided 46 - 46 percent. Backing Democratic candidates are black voters 80 - 13 percent and Hispanic voters 60 - 35 percent.