Regardless of who wins the special House election in Pennsylvania on Tuesday, the Democratic candidate, Conor Lamb, has already accomplished something impressive by showing that his party ought to contest every election — no matter how daunting the odds.

“You miss 100 percent of the shots you don’t take,” goes the line often attributed to the hockey great Wayne Gretzky and quoted in school gymnasiums ever since. It’s a lesson that bears repeating to Democratic Party leaders, who in recent years effectively surrendered many seats to Republicans under the mistaken belief that Democrats had no chance. For example, the party did not bother fielding candidates during the 2016 and 2014 elections for Pennsylvania’s 18th Congressional District seat, which recent polls show Mr. Lamb could narrowly win on Tuesday. While Donald Trump won the district by 20 percentage points, it has a large population of union members and more registered Democrats than Republicans.

Mr. Lamb, a former Marine and prosecutor, has campaigned energetically and made striking gains without much help from his party. The Democratic Congressional Campaign Committee has put up a small fraction of the money that Republican groups are spending. Even if Mr. Lamb loses on Tuesday, analysts say he could easily win in November, when Pennsylvanians will vote under a new congressional map ordered by the State Supreme Court in an important gerrymandering lawsuit.

There is no doubt that Mr. Lamb has benefited from running against a lackluster opponent, Rick Saccone, who even many Republicans have privately grumbled is a “weak” candidate. But Mr. Saccone’s weakness, which now seems obvious, would not have been much of a factor in the race had Mr. Lamb not decided to run in the first place. Without a serious Democratic challenger, Mr. Saccone might have coasted to victory like the last person to hold the seat, Tim Murphy, a Republican who resigned after it was revealed that he had encouraged a woman he was having an affair with to have an abortion even though he publicly opposed the procedure. The same argument applies in the case of the special Senate election in Alabama last year, in which the Democrat Doug Jones triumphed against Roy Moore, a Republican who several women said harassed and abused them when they were teenagers and he was in his 30s.