The U.S. Capitol building in Washington, D.C., on July 19,...

The U.S. Capitol building in Washington, D.C., on July 19, 2017. Credit: Bloomberg News / Andrew Harrer

It is as lasting an American literary metaphor as Captain Ahab and the white whale or Hester Prynne and her scarlet “A.”

We are, of course, referring to that branch of science known as cartoon thermodynamics. The first law, as defined by the late film critic Roger Ebert, is worthy of Isaac Newton: “Any body suspended in space will remain in space until made aware of its situation.”

In layman’s terms, that means that Wile E. Coyote and Daffy Duck can race off a cliff and hover in midair until they make the fatal mistake of looking down.

This is akin to what has occurred in Washington this month - both the Republican and Democratic parties have long been in trouble, but now circumstance has forced them to look down.

Consider the recent effects of political gravity:

-By cutting a debt ceiling deal with Sen. Chuck Schumer and Rep. Nancy Pelosi, Donald Trump marginalized the Republican congressional leadership. The snubbing of Sen. Mitch McConnell and Rep. Paul Ryan gave rise to news analyses like The New York Times story headlined “Bound to No Party, Trump Upends 150 Years of Two-Party Rule.”

-Defrocked White House strategist Steve Bannon is plotting 2018 primary challenges against incumbent Republican senators, including the otherwise secure Bob Corker in Tennessee. What gives heft to these threats is Bannon’s financial backer: conservative billionaire super PAC baron Robert Mercer.

-Recent decisions not to run for re-election by popular House Republicans like Charlie Dent of Pennsylvania and Dave Reichert of Washington are the latest indicators that there is no longer room for old-fashioned moderates in the GOP.

-After displaying impressive unity in the battle to save Obamacare, the Democrats returned to their fractious ways with leaked excerpts from Hillary Clinton’s score-settling campaign memoir, “What Happened.”

The book, which was published Tuesday, contends Sanders’ 2016 attacks “caused lasting damage, making it harder to unify progressives in the general election and paving the way for Trump’s ’Crooked Hillary’ campaign.” Then Clinton adds archly, “I don’t know if that bothered Bernie or not.”

The crusty Vermont senator couldn’t resist answering in kind. In an appearance on CBS with Stephen Colbert, Sanders snipped, “Secretary Clinton ran against the most unpopular candidate in the history of this country, and she lost. And she’s upset about that. I understand that.”

-On another front in the never-ending Hillary-Bernie proxy war, the Democratic Party’s Unity Commission is exploring the role of caucuses and superdelegates during the 2020 campaign.

But, in a move that underscores the weakness of the national parties, the state of California is expected this week to move its primary from June to early March 2020. Not only would such a dramatic alteration in the political calendar give a massive edge to California candidates, it could also effectively disenfranchise most other states in choosing a nominee.

-There was a flurry of recent talk that Ohio Republican Gov. John Kasich and his Colorado Democratic counterpart John Hickenlooper were considering running for president and veep as independents in 2020.

Both governors, who are working together on health care reform, denied such bolt-their-party ambitions. But the rumors illustrated the continuing fascination with a plague-on-both-your-houses independent politics.

Remember, all this has happened in the last two weeks.

A strong case can be made that both Republicans and Democrats stepped off the cliff in 2016 - and only now are we grappling with the full implications of their plunge.

As much as ill-fated former Republican National Committee Chairman Reince Priebus tried to mask it, Trump’s nomination was nothing short of a hostile takeover of the GOP. Of course, Sanders battled Clinton all through the primaries without ever deigning to join the Democratic Party.

The July 2016 publication by WikiLeaks of hacked emails from the Democratic National Committee helped convince Sanders’ supporters that the fix had been in at the DNC. But what few news stories at the time conveyed was how feeble the DNC had become with Debbie Wasserman Schultz as party chairwoman.

In a sense, the rise of super PACs turned both national party committees into afterthoughts.

With independent funders like Mercer and the Koch brothers on the Republican side and free spenders like Tom Steyer for the Democrats, congressional candidates no longer look to the DNC and RNC for a major infusion of funds. What party funds exist mostly flow through the House and Senate campaign committees on both sides.

But it would be a mistake to underestimate the resilience of the two-party system.

If Trump truly had cut his ties with the GOP leadership on Capitol Hill, then congressional Republicans would be taking bold steps at this moment to aggressively investigate the president’s alleged ties to Russia. At minimum, there would be a serious look at whether the Trump family (including son-in-law Jared Kushner) was personally profiting from government service.

All glib talk about independent races for the White House ignore the obstacle that prevented Mike Bloomberg from running in 2016 - the Constitution. As long as a deadlocked presidential race is settled in the House, it is hard for an independent candidate to map a route to victory.

Political parties have endured vicious infighting before. The rise of Barry Goldwater ripped the Republicans apart in 1964 - and the Democrats soon bitterly split over Vietnam.

But the Electoral College and the lack of proportional representation in congressional elections means it will take more than Donald Trump and a few bad years to destroy the two-party system.

Roll Call columnist Walter Shapiro is a veteran of Politics Daily, USA Today, Time, Newsweek and The Washington Post.

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