There is a single word that qualifies Hillary Clinton for the nomination: experience. Ms. Clinton may be the most uniquely qualified person to run for the presidency in a generation. Her work as secretary of state gave her firsthand experience understanding complex foreign policy issues and dealing with allies and adversaries. Her service as New York’s junior senator demonstrated that she understood how to leverage federal agencies, pierce the byzantine budget process and find support from Republicans to secure essential funds for New York after 9-11. And her perspective gained as first lady—to say nothing of the bruising battle to reform health care—was invaluable. It is difficult to think of another candidate with such varied and valuable experience.

It is important to address Mr. Sanders’ qualification and vision as well. Mr. Sanders is a one-issue candidate: All ills will be solved by taxing Wall Street and breaking up large financial institutions. Beneath this populist screed, however, is not just economic theory but years of economic experience—which unfortunately, Mr. Sanders appears not to have learned. Mr. Sanders is too disingenuous—and his young supporters too bereft of substantive historical knowledge—to remember that socialist theories simply have not worked. The inevitable outcome of Mr. Sanders’ policies would be to Make America Greece Again.

The greatest difference between Ms. Clinton and Mr. Sanders is in their respective approaches to foreign policy. We may not agree with much of President Obama’s foreign policy—we have editorialized forcefully against much of it—but Ms. Clinton’s experience at the crucible is invaluable. She recognizes that the world is a dangerous place. Sometimes we can and must forge alliances to pursue our national interests; other times there is a need for solitary, difficult leadership.

Hillary Clinton understands “the system”—our political brinksmanship—and how to work within it to disrupt it.

Mr. Sanders’ inadequate grasp of these complexities—and the challenges of Realpolitik to deal with them—have been on display throughout the campaign. We were particularly disheartened several weeks ago when Mr. Sanders sat down with Judy Woodruff of the PBS NewsHour for a one-on-one interview. When she pressed him—far too gently—to explain his “plan to defeat ISIS,” his response was embarrassing. His oversimplification and wishful thinking demonstrated a lack of understanding that was inexcusable.

This wasn’t a one-time deal. In an interview with the Daily News, Mr. Sanders accused Israel of “disproportionate” action during the 2014 war in Gaza, based on his “recollection” that more than “10,000 innocent people were killed in Gaza.” The actual number of Gazans killed was about 2,100, but even more troubling than the 400 percent exaggeration—all candidates misrecall exact numbers when speaking extemporaneously—was the worldview that provoked it. Mr. Sanders, despite affirming Israel’s right to exist in safety, clearly favors the ugly “Blame Israel Automatically Always” narrative that is so popular among the same campus-based progressives who comprise his base.

We recognize, however, that even a broken clock is right twice a day. Mr. Sanders’ comments about the need for greater scrutiny of trade deals are valid. And his complaints about how the primary-and-delegate system is convoluted, illogical and grossly unfair are correct. Both sides are seeing the will of the people overturned by party bosses, and the American people are experiencing a rare moment of bipartisan disgust. When Bernie beats Hillary 56-44 in Wyoming only to see her walk away with 11 delegates to his 7, his supporters are justified in being just as angry as Republicans who wonder how all of Colorado’s 34 delegates wound up in Ted Cruz’s column without even a vote.

Unfortunately, Mr. Sanders’ candidacy reminds us of Norman Thomas’: one note and slightly off-key. Norman Thomas may be forgiven his disingenuousness, as he ran as a socialist before the savage truth of its failures were out there for all the world to see. Bernie Sanders is old enough to know better.

Ms. Clinton is not without her own challenges. Her email scandal and the solicitation of contributions by the Clinton Foundation while she was secretary of state linger like an unwanted guest to the Lincoln bedroom at the White House. But we are willing to accept—to hope—her assurances that these clouds are without substance and will soon dissipate.

Far more importantly, we believe that Ms. Clinton shares our “New York values,” a belief in the opportunities in this great melting pot, in gay rights, in concern for the environment. Mr. Sanders may be a New Yorker by birth, but he chose to leave. Ms. Clinton is not a New Yorker by birth; she chose our home as her home. His experience and perspective are of Burlington and Vermont; while many of the most pressing challenges facing the next president will be those facing New York and America’s other great cities. Bernie comes from a white, rural state that looks less and less like the rest of America. The simple truth is he was late to many issues—especially gun violence, which is so important here. And despite his this holier-than-thou character, he voted in favor of guns as a senator from a rural state because his constituents like guns. Which is fine. But it shows he too can be swayed by the masses—and now he’s been swayed back in the other direction.

Ms. Clinton was shrewdly effective in helping to rebuild and secure this great city after 9-11 when she and Sen. Charles Schumer led the fight for funds. Members of the Observer’s editorial board personally saw her spend hours at Ground Zero comforting rescue workers, and she was among the first to raise alarm bells about the possible toxicity of the piles of smoldering debris. Her appreciation for the ongoing need for adequate funding to rebuild essential infrastructure is an important consideration in our endorsement. America needs a significant investment to upgrade our bridges, water systems and air-traffic control systems—to name just a few priorities—not just for safety but also for enhanced productivity and fuller employment. The economy will benefit; people will benefit; and Ms. Clinton understands this. Yesterday during a visit to Al Sharpton’s National Action Network, she talked at great length about plans to address crumbling infrastructure like public housing but also unseen problems like lead pipes. That’s Hillary at her best—the compassionate, thoughtful leader who’s done her homework.

As first lady, Ms. Clinton was one of the first and most articulate champions of early childhood education. Her support for research-based, cost-effective initiatives, like the pediatrician-led Reach Out and Read (ROR), showed an understanding of the need to go beyond traditional Democratic models and adopt cross-disciplinary innovations—here, combining health care and education. That Laura Bush subsequently endorsed the ROR initiative when she became first lady is significant. It was a dramatic departure from the tradition of a new administration abandoning her predecessor’s priorities and is a testament to both women: to Ms. Clinton for crafting a program that could gain Republican support and to Ms. Bush for recognizing the importance of progress over partisanship.

Crossing the aisle is too infrequent today and both the cause of and result of gridlock. Hillary Clinton understands “the system”—our political brinksmanship—and how to work within it to disrupt it. Bernie Sanders, whose ideas are occasionally noble, chooses to believe it can be kicked over like a sand castle. We like to believe she learned some of her pragmatism (some call it “cynicism”) as a New York politician and from the history of our politics. Play the long game, work the system, be an operator, not a hand-wringer. Dewitt Clinton, Al Smith, Robert Wagner, Shirley Chisholm, Bella Abzug and even Daniel Patrick Moynihan would all approve. We believe that Ms. Clinton appreciates and will be able to revive that spirit of cooperation. It is with that hope and belief that we enthusiastically endorse Hillary Clinton in the Democratic primary on Tuesday.