All 15 Democrats on a House committee boycotted last week’s field hearing on the crisis at our southern border, aptly held in Texas for the express purpose of focusing lawmakers’—and national—attention on out-of-control illegal immigration.

Worse, the March 15 hearing of the House Homeland Security Committee in Pharr, Texas, marked the second time in less than a month that House Democrats irresponsibly refused to attend an immigration-related hearing held on the U.S.-Mexico border.

Three weeks earlier, on Feb. 24, all 19 Democrats on the House Judiciary Committee likewise skipped a similar hearing in Yuma, Arizona.

Equally irresponsibly, both hearings also went largely unreported by the mainstream media.

House Democrats’ knee-jerk refusal to participate in those hearings was nothing short of a dereliction of duty, especially considering it involved an issue of such transcendent importance as our national sovereignty.

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The Republican majority on the committee somewhat provocatively—but not inaccurately—dubbed the hearing “Failure by Design: Examining Secretary Mayorkas’ Border Crisis.” That would be Alejandro Mayorkas, President Biden’s hapless secretary of homeland security.

Overwhelming evidence—namely, hundreds of thousands of illegal immigrants a month streaming across the border—to the contrary notwithstanding, Mr. Mayorkas continues to insist that the border is “secure,” and that the Department of Homeland Security has “operation control” of it.

The committee’s announcement of the hearing described its purpose as “outlining how the crisis at the Southwest border is a direct result of Secretary Mayorkas’ failure to enforce the laws of our country.”

That pointed criticism of Mr. Mayorkas’ apparently intentional failure (for which he deserves to be impeached and removed from office) might have given the Democrats the cover they needed to boycott the hearing. But it was unarguably accurate, and to borrow the memorable line from Jack Nicholson’s Marine character, Col. Nathan R. Jessup, in the 1992 film “A Few Good Men,” it appears that Democrats “can’t handle the truth.”

At last week’s hearing, the AWOL Democrats missed out on a chance to hear the unvarnished truth about the border crisis from those on the front lines of the invasion: U.S. Border Patrol Chief Raul Ortiz; Kinney County, Texas, Sheriff Brad Coe; National Border Patrol Council Vice President Chris Cabrera; and Col. Steven C. McCraw, director of Texas’ Department of Public Safety.

The Democrats’ indefensible absence raises an uncomfortable question: Is it because of reflexive allegiance to the Biden administration and, by extension, to Mr. Mayorkas, that Democrats are willing to turn a blind eye to the border crisis? What better place to see the true nature of the problem for themselves? Isn’t that the whole point of congressional fact-finding trips?

As pre-Shakespearean poet-playwright John Heywood observed more than 475 years ago: There are none so blind as those who will not see.

As such, Democrats should change their party’s symbol from the donkey—nowadays more like the proverbial Missouri mule, albeit even more obdurate—to either an ostrich with its head buried in the sand or to the “See No Evil, Hear No Evil, Speak No Evil” monkeys.

That prompts us to ask: When will someone—anyone—in the GOP pointedly demand answers from Democrats to these questions about the ongoing flood of illegal immigration: How many is too many? Is it 10 million, 25 million, 50 million? Indeed, we’re left to wonder: At what point will Democrats finally say: “No mas!”?

In that same vein, when will that ubiquitous estimate of 11 million illegal immigrants in the country that has been accepted as fact for close to 20 years be subjected to fresh examination and updated?

The Office of Immigration Statistics at DHS asserted in December 2018 that as of Jan. 1, 2015, there were 11.96 million illegal immigrants in the U.S. A Pew Research Center estimate in 2017 put the total number of illegals in the country at 10.5 million.

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But those numbers were roughly the same as what both DHS and Pew’s Hispanic Center were citing 10-plus years earlier, in 2005. Does anyone really believe there was no net increase in immigration in the interim?

In fact, many argued the actual number of illegals in the U.S. was far more than 11 million even then—and that was long before Mr. Biden and Mr. Mayorkas effectively threw open the floodgates in Jan. 2021.

The conservative One America News Network in July 2019—extrapolating from data from several sources, including the DHS itself—estimated the “total illegal aliens in U.S.” at 26.43 million. Conservative firebrand author Ann Coulter, in her 2015 book “Adios, America,” estimated the number at closer to 30 million.

As a practical matter, however, the actual number of illegal aliens in the country is not only unknown, but unknowable, so any rational congressional debate about illegal immigration and what to do about it must begin with an honest assessment of the true size and scope of the problem.

The GOP majority in the House also needs to task the Government Accountability Office (or some other federal agency capable of quantifying it) with estimating not only the number of illegal immigrants in the country, but also calculating how much—surely in the tens of billions of dollars annually—their presence in the country is costing the states and the federal government in the cost of education, health care, incarceration and more.