The first question I posed was: Could Lamb — who did not face a primary — have won the seat if the contest had been held in 2016, during a general election? The answer was no:

We know that PA-18 was not winnable in a year with Hillary at the top of the ticket. Democrats did not win any seat with the demographic profile of this one anywhere in the country in 2016 — save for handful of longtime incumbents.

How about 2020? Could a centrist like Bill Clinton get the nomination?

I’d say the answer is “no.” I think the politics of the Democratic Party have changed significantly since that time and while it may not be the MOST liberal candidate who wins the nomination, all viable candidates will have to pass a certain threshold of liberal-ness.

One of the best case studies of center-left intraparty schism can be seen in the upcoming May 22 runoff election for the Democratic nomination in the 7th Congressional District in Texas — upscale suburban Houston.

The two candidates are Lizzie Pannill Fletcher, a local lawyer who led the primary with 29.3 percent and is generally viewed as the moderate, establishment candidate (referred to as Republican-lite by her opponents), and Laura Moser, a liberal political activist, author and former journalist, who came in second with 24.3 percent, and was described by the Houston Chronicle as the candidate of “those who want a full-fledged push to the left to provide the starkest possible contrast with Trump.”

The Democratic Congressional Campaign Committee, convinced that Moser would be a weak general election candidate, turned the primary contest into an internecine clash when it released a document on Feb. 22 designed to damage Moser’s bid:

She is a Washington insider, who begrudgingly moved to Houston to run for Congress. In fact, she wrote in Washingtonian magazine, “I’d rather have my teeth pulled out without anesthesia” than live in Texas. As of January 2018, she claimed Washington, D.C. to be her primary residence in order to get a tax break.

The attack by the D.C.C.C., viewed by many in the liberal wing of the party as aggressively centrist, generated a backlash, mobilizing the left in support of Moser. She now has endorsements from Howard Dean’s group, Democracy for America, Bernie Sanders’ Our Revolution and the insurgent Justice Democrats.

“The D.C.C.C.’s ridiculous attacks on Laura Moser are why Democrats nationally have lost over 1,100 seats. Laura is a rising progressive advocate that the workaday people of Texas desperately need,” Jim Hightower, a board member of Our Revolution, said. “She is an active organizer who takes on the Powers That Be on behalf of the Powers That Ought To Be — workers, consumers, small businesses, and just plain folks.”

The district has been in Republican hands for 52 years, since George H.W. Bush first won the seat in 1966. In the 2016 election, however, Hillary Clinton beat Trump there, 49-47, abruptly putting the 10-term Republican incumbent, John Culberson, high on the Democrats’ target list.

Whoever wins the runoff, the general election will provide a test of Democratic competitive strength in what could be called Hillary districts — Republican-held seats where she did well. These districts are relatively affluent — in the case of the 7th District, the median household income is $71,183, substantially above the national median of $57,617 and the Texas median, $54,727; and well-educated — 50.7 percent with a college degree or higher, compared to 30.3 percent nationwide and 28.1 percent in all Texas.

To bring this full circle, the upscale 7th District in Texas and the downscale 18th District in Pennsylvania effectively represent the dilemma of a party increasingly dominated by cultural liberals.