Marie Newman, a political newcomer who is challenging Rep. Daniel Lipinski (D-Ill.) in the March 20 primary. (Sara Burnett/AP)
Opinion writer

In 1964, as the National Review was trying to determine which Republican to support for president, the magazine’s leader, William F. Buckley Jr., articulated what would become known as the Buckley Rule: They would support “the rightwardmost viable candidate.” While “viable” didn’t necessarily mean “electable” in the way we now understand it, that’s how the rule is remembered, as a balancing of ideology and practicality.

Now something similar is happening among progressive Democrats. A number of incumbent Democrats are being targeted with primaries from the left. And this development is being widely seen through the prism of the Bernie Sanders/Hillary Clinton primary fight, with some wringing their hands about how the left is becoming the new tea party and about how destructive this will be to the Democrats’ chances.

But in fact, the effort from the left, on the whole, is quite restrained, and arguably could have a positive impact on the party over the long term. Indeed, the left is following its own version of the Buckley rule — and while there is a lot of anger at President Trump, we haven’t seen the kind of enraged revolt against the party’s own leaders that took hold of the GOP in 2009.

Let’s take a look around. We have seen some high-profile primary challenges, but they really reflect the fact that 2018 is a year in which the party’s base is particularly energized. So this is the time for liberals to rid themselves of someone like Rep. Daniel Lipinski of Illinois, one of the most conservative Democrats in the House (he opposes abortion rights and voted against the Affordable Care Act, among other things) who represents a district Hillary Clinton won by 15 points. While Lipinski retains House Minority Leader Nancy Pelosi’s support, his opponent, Marie Newman, is backed by many parts of the Democratic establishment, such as Emily’s List and the Illinois chapter of the Service Employees International Union, in addition to prominent Democrats such as Sen. Kirsten Gillibrand (N.Y.) and Reps. Luis V. Gutiérrez (Ill.) and Jan Schakowsky (Ill.).

In other words, while Lipinski warns that the opposition to him represents a self-destructive “tea party of the left,” the bid to unseat him actually seems like a low-risk strategy to improve the ideological unity of the party’s representatives in Congress. If Lipinski represented a red district, would he be getting the same kind of strong challenge? Probably not. You can say the same about Sen. Dianne Feinstein of California, who is being challenged by state Sen. Kevin de León. At the moment this looks like the only strong primary challenge to any of the 25 Senate Democrats up for reelection in 2018, and it’s happening in a state all but guaranteed to elect a Democrat. There’s a strong argument that Feinstein is far too conservative for the state she represents, not to mention the fact that if she wins, she’ll be 91 years old at the end of her next term. But de León’s candidacy doesn’t really risk anything.

One more example: If liberals were imposing rigid ideological purity tests, they wouldn’t be supporting Conor Lamb, who is running in a special election that takes place next week. Trump won this southwestern Pennsylvania district by 20 points, and Lamb is the kind of Democrat who could win there even if he isn’t a liberal’s dream: He’s pro-union but pro-gun (though he supports improving background checks); he’s “personally opposed” to abortion but pro-choice as a matter of policy; and he supports shoring up the Affordable Care Act rather than moving to single-payer. Lamb has raised a huge amount of money, mostly in small-dollar contributions, and there are few if any liberals decrying his nomination, because they realize he’s about the best Democrats can do in a district like that.

The Democratic Party as a whole is undoubtedly moving left, and there are certainly going to be candidates in some places who test the willingness of general-election voters to support liberal positions. But there are far fewer primary challenges than you might have expected, and the point doesn’t seem to be making Democratic representatives terrified of straying from dogma the way the tea party did to Republicans.

Which brings us to the question of whether this kind of pragmatism on the part of the left is the best strategy. The answer depends in part on how you view what the tea party accomplished. On one hand, they certainly took over the GOP and made the establishment quake in fear, particularly after primary challenges ousted people such as Rep. Eric Cantor (Va.) and Sen. Robert Bennett (Utah). They imposed an iron discipline on Republicans in Congress, making opposition to President Barack Obama absolute. And even though they looked extreme in their beliefs and radical in their tactics, that didn’t stop Republicans from taking the House in 2010, the Senate in 2014 and the White House in 2016.

On the other hand, the tea party also cost the GOP many seats it could have won, when it nominated extremists and cranks who eventually revealed themselves as such and lost; you may recall Todd Akin, Sharron Angle, Richard Mourdock and Christine O’Donnell. And the GOP has now locked itself into a version of angry white identity politics that may have prevailed in 2016 but will be increasingly unhelpful with each passing year.

In other words, the tea party struggled to find the right balance between ideology and practicality, because it convinced itself that maximalism was always the best strategy. At the moment it looks like Democrats are steering a more pragmatic course. It might leave them with a few more moderates in their caucus next year, which could make opposing Trump more complicated. But it could also help them win the House — which would make it all worth it.