SE Cupp is a CNN political commentator and the host of " SE Cupp Unfiltered ." This piece has been adapted from her Saturday evening show monologue. The views expressed in this commentary are solely hers. View more opinion articles on CNN.

(CNN) Defending the indefensible has become a cottage industry for Republican lawmakers in the era of Trump.

In the days and weeks surrounding President Donald Trump's decision last year to enforce the separation and caging of asylum-seeking children at the southern border, Republican lawmakers were largely supportive of him.

In the days after a baffling news conference last year in which President Trump, standing alongside Vladimir Putin, parroted the duplicitous Russian position that it did not meddle in our elections -- even though his own intelligence community had said the opposite -- Republican lawmakers shrugged.

In the days and weeks after the release of the Mueller report earlier this year, which revealed multiple examples of the President's attempts to engage in obstructive conduct, Republican lawmakers were defiantly behind him.

And in the days after the news about President Trump's alleged attempt to coerce the Ukrainian president to investigate his political rival (Trump denied that was his purpose) -- news that unleashed a stunning flood of evidence, transcripts, testimony, texts, and whistleblowers and ultimately resulted in an impeachment inquiry -- the GOP circled the wagons around their President.

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Those are just a few of the many embarrassing episodes from the past few years where Republican lawmakers had opportunities to condemn the indefensible, and many have chosen not to. Many, in fact, have decided to abandon their conservative principles, their ethical and constitutional obligations -- not to mention their sense of common decency -- to justify Trump's odious behavior and deleterious decisions.

One issue, however, has proven surprisingly perilous for the President, a rare pressure point that has caused Republican lawmakers to summon courage and roundly condemn him: Syria.

While President Trump hopes the announcement Thursday of a 120-hour ceasefire in Turkey, which Turkey says is "not a ceasefire," might appease his many detractors, much of the damage is already done.

In the days after President Trump's disastrous, immoral, inexplicable decision to pull US troops out of Kurdish-controlled Northern Syria, unleashing the Turkish army on an overwhelmed US ally, sending untold numbers of ISIS prisoners back into the Sahel to reorganize, and handing some of the world's worst actors the keys to a broke-down and dangerous palace, Republicans were quick to voice their disapproval.

Even his staunchest allies, including South Carolina Sen. Lindsey Graham, have freely rebuked him. Graham told an NBC News reporter that he would "become President Trump's worst nightmare" on Syria if he didn't reverse course: "This is a defining moment for President Trump. He needs to up his game."

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It wasn't the first time he's drawn this reaction: When Trump first threatened to pull troops out of Syria in late 2018, defying his generals and security advisers, Republicans were stunned. Graham told reporters, "If Obama had done this, we'd be going nuts right now: how weak, how dangerous."

He and other Republican lawmakers, including Sens. Tom Cotton, Marco Rubio and Joni Ernst, signed a letter telling Trump to reconsider.

That episode, in fact, led two of Trump's most important figures in the Syria conflict, Gen. James Mattis and Brett H. McGurk, the American envoy to the coalition fighting the Islamic State, to resign their posts.

And now, as a quickly spiraling disaster in Northern Syria grows worse, Republicans have come together again in a rare reproval of the President. The House of Representatives voted Wednesday to condemn Trump for pulling troops out of Syria to allow for a Turkish invasion by a vote of 354-60, with 129 members of his own party voting in favor of the measure.

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It's hard not to notice the glaring singularity of Syria as an issue that, sui generis, unlocks the Republican caucus from Trump's otherwise vise-like grip.

One cynical explanation for this is that foreign policy issues are usually a safer space for dissent, at least in the short term. Lawmakers assume constituents back home are more concerned about immediate and pressing domestic issues, especially during an election cycle, and many are likely gambling that Trump isn't going to unleash his primary attack apparatus against them over a Syria disagreement.

But the other explanation is that the consequences of Trump's impulsive, ill-informed, politically craven and incomprehensible decision to abandon our Kurdish allies, empower Turkey's Erdogan and Syria's Assad, dissolve our containment of ISIS and put hundreds of thousands of lives in the balance are just too much to stomach for Republican lawmakers.

They have little to gain at home for condemning Trump's actions overseas. Voters are generally apathetic to foreign policy issues. In a Gallup poll from earlier this year that asked what voters think is the most important problem facing the country today, issues like immigration, health care, gun crime and the environment led the lists. Foreign policy got just 1% of the vote, and both ISIS and Russia received 0%.

So the rebuke of Trump wins Republican lawmakers no points in their own districts, at least in the immediate future.

But the long gaze of history is far less forgiving.

When the fog of war clears, voters do tend to hold major foreign policy blunders against elected officials, even in their own party. See: the Vietnam, Iraq and Afghanistan wars. Republicans know that the actions Trump is taking today in the Middle East could haunt him and the GOP for years to come.

And it's not just the folly of war they're worried about. It's the significant humanitarian crisis this will manifest, adding to the one that already exists in that region. There's the long-term threat to our own national security when ISIS and other terror actors regain a footing in Europe and even here at home. There's the destabilizing effect this has on important US allies like Israel. And there's the breakdown of trust among our allies all over the world.

All of that is -- right now -- staring GOP lawmakers in the face.

It should tell us something that Republicans, who are usually so protective of this President, despite an ever-crescendoing wave of bad behavior and bad decisions, have spoken out so vocally and unilaterally against him when it came to Syria. That's how fraught, how devastating and potentially disastrous this issue is both politically and practically.

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And that's a problem for Trump. Because if he decides not to listen, it's an excuse for Republican lawmakers, who may have secretly been looking for an opportunity, to break ranks, and at the worst possible time for the President -- when impeachment is actually on the table.

For all those reasons, Trump should be extremely concerned. He doesn't like to be told no, and he doesn't like defectors. But on this issue, perhaps more than any other, he would be wise to listen to the majority of his own party telling him to stop. Otherwise, it may just spell the end for his presidency.

Correction: An earlier version of this commentary incorrectly stated the duration of the ceasefire in Turkey announced Thursday. It is a 120-hour ceasefire.