Some of the most zealous "Never Trump" conservatives are seeking a candidate to challenge President Trump in the 2020 Republican primaries. It's an uphill slog in GOP politics, where the incumbent president enjoys substantial support and huge tactical advantages. But the Never Trumpers' task could be further complicated by developments on the Democratic side of the race.

The dilemma: A primary challenge could weaken Trump and ensure the election of a Democrat. And as the Democratic Party lurches left, that could mean the work of "Never Trump" conservatives ensures the election of a radical candidate on a platform of the "Green New Deal," tax rates above 70 percent, universal healthcare, free college, even more liberal abortion policies, and more. It would be hard to imagine a more self-defeating result for activists who previously worked in conservative Republican politics.

This is not a secret. "Never Trump" strategists are well aware that primary challenges weaken sitting presidents and help the opposition party. Some strategists are actually reminding potential supporters of that fact as they seek to organize a challenge to Trump. In 1976, President Gerald Ford faced a primary challenge and went on to lose the election. In 1980, President Jimmy Carter faced a primary challenge and went on to lose the election. In 1992, President George H.W. Bush faced a primary challenge and went on to lose the election.

Of course, there were other factors involved, particularly in the post-Watergate year of 1976. But in none of those cases did the primary challenger — Ronald Reagan, Ted Kennedy, and Pat Buchanan — go on to win his party's nomination that year.

Sarah Longwell, a Republican strategist working with former Weekly Standard editor Bill Kristol in an anti-Trump group called Defending Democracy Together, recently cited another example from history. "In 1968, incumbent President Lyndon B. Johnson — supposedly on a glide path to the Democratic nomination — was stunned to win barely 50 percent of the vote in the New Hampshire primary against Sen. Eugene McCarthy's 42 percent," Longwell wrote in a Feb. 1 Washington Post op-ed. "Johnson dropped out of the race."

Longwell neglected to mention that McCarthy did not go on to win the Democratic nomination in 1968. The man who did, Vice President Hubert Humphrey, was defeated in the general election. But Longwell's "Never Trump" readers cannot have failed to get the message: Challenge Trump, and he will lose.

Last month, the New York Times reported that Longwell and Kristol "have been meeting with Republican donors and potential challengers, sharing focus-group and polling data about the president's vulnerability with what they call 'Reluctant Trump Voters.'"

If "Never Trump" politicos persuade a reasonably well-known Republican — Maryland Gov. Larry Hogan, former Ohio Gov. John Kasich, or former Massachusetts Gov. William Weld, who has just launched an exploratory committee — to challenge Trump in the GOP primaries, the likely result will not be that the party will nominate Hogan or Kasich or Weld but that it will nominate an incumbent damaged by the primary battle.

"I think any primary challenge of Trump is doomed to fail and silly to even start," Republican strategist Brad Todd told me in a recent email exchange. "Trump was the left-most candidate in the 2016 primary, and now his record is more conservative than his platform was then. You don't beat Republican incumbents in primaries from the left. The question is whether NeverTrumpers are willing to get it out of their system via a primary challenge. If they run a primary and lose, will they unite to beat the socialists in November? That is the question we should be asking."

Another GOP strategist, David Carney, suggested that Trump has vulnerabilities, while at the same time Never Trumpers are not the ones to exploit them.

"The tone and tenor matter more than just running," Carney said in an email exchange. "The problem with the NeverTrumpers is they never gave Trump a chance and are perceived as being negative, on the whole, about everything Trump does or says. Primary voters will think they were not 'team members.' Someone else, if they run a serious effort and not just for media attention, could have a shot."

Perhaps. Never Trumpers point to a few polls from early voting states to show that Republican voters would be open to a primary challenge to Trump. Longwell noted a Des Moines Register poll showing 63 percent of registered Republicans in Iowa would be open to a challenge, and a New Hampshire Journal poll showing 40 percent of GOP voters think a challenge would be a good thing.

At the moment, such polls are the Never Trumpers' best case for a challenge. But there are doubts about whether the surveys really say much, because voters generally don't like to tell pollsters that their minds are closed a year before an election. "At this early point in any campaign, people are not going to shut their minds," said Republican pollster David Winston in a recent conversation. "They just don't do that. There's a huge difference between people saying they're thinking through things and how they behave. My guess is those polls are probably true, but that's because the electorate is trying to be thoughtful."

Winston believes the Democratic move to the left will change the nature of the 2020 race. The more Democrats advocate policies that were on the party's far left just a short time ago — and are still out of the larger political mainstream — the more the race will become a referendum on the future of the country, on which way it should go, and not on the personalities of the candidates. That could unite Republicans — and many independents — against moving in a policy direction so antithetical to their beliefs.

An example of that, Winston noted, is the recent progressive Democratic opposition that forced Amazon to pull out of a plan to build a second headquarters in New York City. The Amazon opponents, Winston wrote in Roll Call, "showed the American people the real-world consequences of progressive socialism and gave Republicans the perfect example to illustrate the choice in 2020 — radical economic policies that cost jobs or pragmatic, commonsense economic policies that drive growth and create jobs."

But maybe there is some other way a GOP Trump challenge might work. Maybe the Democratic Party will shift again and nominate former Vice President Joe Biden as a more centrist candidate. It's possible, although it seems hard to believe Biden could win the nomination without moving steadily left. In any event, Never Trumpers could cite a "moderate" Biden presidency as an acceptable outcome of their challenge to Trump.

That is theoretically possible, but the fact is, the 2020 choice is likely to be between Trump and a sharp turn left. And if that comes to pass, Never Trumpers challenging the president — on the explicit grounds that the challenge will weaken him and cause him to lose — would be a key part of that sharp turn left.

There is yet one more possibility. Some Never Trumpers could be betting that a damning report from Trump-Russia special counsel Robert Mueller will fatally weaken Trump and possibly lead to impeachment and even removal from office, sending Republicans scrambling for another candidate. In that case, it is probably safe to say that the GOP would be screwed whatever it does. A "Never Trump" challenge would be just one element of an extremely ugly post-Mueller intraparty battle.

Short of that, though, it seems likely that Republican voters will stick with the Republican president. And that is even more likely if Democrats keep moving left. Yet some Never Trumpers, long committed to "disposing of" Trump (Kristol's phrase), will no doubt keep trying, no matter where that might lead.