During his presidential campaign, Donald Trump often complained about how President Barack Obama drew red lines that he never enforced, and how a diminished America let Russia walk into Syria unchallenged, something he said would not happen if the Russian leader respected the United States president.

Now, in Venezuela, President Trump is facing his own red-line moment — again, with Vladimir V. Putin of Russia.

For the past week, the Trump administration has escalated its warnings about Russian intervention in the country, claiming that Moscow is helping to prop up President Nicolás Maduro of Venezuela and undermining the hopes of American officials that the Venezuelan military will oust him.

The United States recognizes Juan Guaidó, the opposition leader, as Venezuela’s rightful president.

On Friday, John R. Bolton, Mr. Trump’s national security adviser, said that the arrival of roughly 100 Russian military “advisers” in the country, along with new arms, posed “a direct threat to international peace and security in the region.” Mr. Bolton warned that no country should enter the Western Hemisphere “with the intent of establishing or expanding military operations.”

That seemed to set up a test: Would Venezuela be the place where Mr. Trump, who has often seemed willing to tolerate Mr. Putin’s most audacious provocations, finally draws his own red line? And, if so, does he have a plan to enforce it?

It is notable that Mr. Bolton, not Mr. Trump himself, made the announcement, with its echoes of the 1823 Monroe Doctrine, the first time the United States issued similar warnings telling foreign powers not to intervene in the Western Hemisphere.

Yet, Mr. Maduro is digging in — despite the rising economic sanctions against his government and the recognition of Mr. Guaidó as interim president by the United States and more than 50 other nations. And Mr. Putin may well see an opportunity to replay Syria in Venezuela, propping up another leader that many American officials said had to go — Mr. Maduro instead of President Bashar al-Assad of Syria, this time — and frustrating Washington’s regional goals.

The administration so far has been quite cautious when it comes to threatening military action in Venezuela. While there have been ritual reminders that “all options are on the table,’’ there is no indication that any military intervention — which has a long and unhappy history in Latin America — is being seriously contemplated.

Instead, the administration is beginning to talk about a long battle of attrition. After the Maduro government declared last week that Mr. Guaidó could not hold public office for 15 years, Elliott Abrams, the State Department’s special representative for Venezuela and a veteran of many Cold War battles in Latin America, told reporters, “I don’t imagine that Juan Guaidó is deeply worried because the Maduro regime, while it might be around in 15 days, is not going to be around in 15 years.”

[Why is Russia helping Venezuela?]

He then talked about tightening sanctions on oil sales, dismissed Russian accusations that the United States was attempting to mount a coup against Mr. Maduro and warned that it “would be a mistake for the Russians to think they have a free hand here — they don’t.” But he carefully avoided explaining why Mr. Putin might feel under constraint.

In fact, Mr. Putin has plenty of reasons for keeping Mr. Maduro in power. Russia is also focused on recovering billions in debt that Mr. Maduro owes Moscow and Beijing, some of which is measured in oil. Full repayment may never happen.

A Russian military plane recently arrived in Venezuela. The Trump administration has escalated its warnings about Russian intervention in the country.CreditYuri Cortez/Agence France-Presse — Getty Images

For Mr. Trump, this is the newest iteration of his many Russia problems — and the latest example of his reticence to talk publicly about Moscow’s malign influence around the world. His silence is jarring, since Mr. Bolton, Secretary of State Mike Pompeo and Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, talk about it almost every week.

They have all embraced the administration’s year-old national security strategy, which recognizes that a resumption of great-power competition, not terrorism, is the primary challenge facing the United States today. Mr. Trump’s former aides say, however, that he never read the full national security strategy, and they note that when he does talk about it, he almost never discusses Russia’s behavior.

Perhaps that is a reaction to the enormous, bipartisan criticism he received after his seemingly deferential approach to Mr. Putin in Helsinki last summer, where Mr. Trump once again appeared to be convinced by the Russian leader’s denials that he had interfered with the 2016 United States presidential election.

Or perhaps it is rooted, as one of Mr. Trump’s aides suggested recently, in a desire not to create the kind of public sound bites that Mr. Obama did — and that he came to regret.

It was Mr. Obama who declared in August 2011 that “for the sake of the Syrian people, the time has come for President Assad to step aside.” And it was Mr. Obama who warned that “the use of chemical weapons is a game changer,’’ then elected, once the chemical weapons were employed, to pull back from military action. Just a year ago, Mr. Trump was still citing that decision, declaring in a tweet that “If President Obama had crossed his stated Red Line in The Sand, the Syrian disaster would have ended long ago.”

The president is not alone in his critique of how the Obama administration handled Syria. One of the most thoughtful analyses of what went wrong came from William J. Burns, Mr. Obama’s deputy secretary of state, in his new book on American diplomacy, “The Back Channel.” It is rife with warnings that could apply to Venezuela as well.

Mr. Burns agreed with what he called Mr. Obama’s “‘long game’ calculus,’’ which included “having the discipline to avoid getting sucked into another military entanglement in the Middle East, which would likely only underscore the limits of our influence in a world of predators for whom Syria’s battle were existential.”

But he admitted that the Obama “short game” was lacking because “we misaligned ends and means, promising too much, on the one hand — declaring ‘Assad must go’ and setting ‘red lines’ — and applying tactical tools too grudgingly and incrementally, on the other.”

Reached in London over the weekend, Mr. Burns praised the Trump administration for avoiding military entanglements that are bound to raise cries of American imperialism, even among Mr. Maduro’s greatest opponents. But he said it was at risk of repeating some of the Obama administration’s mistakes.

“The focus on building political and economic pressure is sensible,’’ he said. And he noted that the geographic distance between Russia and Venezuela limited Mr. Putin’s ability to affect events. “Proximity matters,’’ he said. “They don’t have a way to project their power in ways they demonstrably did in Syria.’’

As a result, much hinges on Mr. Trump’s ability to build a hemispheric coalition, a move that is being undermined, Mr. Burns argued, by the tension with Mexico, especially with Mr. Trump’s recent threats to close the border and to end assistance to El Salvador, Guatemala and Honduras.

Mr. Maduro, too, may be learning some lessons from Mr. Assad, he said. “As long as you control the guys with the guns, even the most unpopular and deeply repressive regimes have the capacity to hang on a lot longer than we might assume.”