Conservatives suspicious of President Trump, along with other administration critics, have wondered if there's any line the president might cross that would meet resistance from the GOP Congress.

Gently but firmly, Congressional Republicans are rebuffing Trump on Russia.

Trump, an unconventional Republican with provocative communication habits, has faced few roadblocks from his more traditional, conservative allies. The Maginot Line appears to be Trump's apologetic, lenient approach toward Russia and strongman Vladimir Putin.

The resignation of Mike Flynn, Trump's national security advisor, partly over potentially improper contacts with Russia, has reinforced their resolve. Top Republicans are demanding an explanation, some even pushing for Congressional investigations while also using the situation to propose slapping Moscow with new economic sanctions.

"We're just being very realistic about the fact that Vladimir Putin is an adversary, and we have to stand up to him," Sen. John Hoeven, R-N.D., said in an interview with the Washington Examiner.

"Trump says unconventional things; a lot of Republicans will roll with it," added a Republican strategist with relationships on Capitol Hill, requesting anonymity in order to speak candidly. "But when he does things that are counter to our orthodoxy on foreign affairs, he's going to hit a wall."

Republicans have let Trump slide on a range of issues, both ideological and political: Business ties that could violate the Constitution's emoluments clause; protectionist trade policies and bullying of U.S. corporations; not to mention his habit of sounding off on Twitter.

But Republicans have been urging Trump to get tough with Russia, which continues to provoke U.S. military assets and undermine American interests. They have warned Trump against getting fooled by Putin, as predecessors President George W. Bush and President Barack Obama were.

Some Republicans have openly chastised the president for excusing Putin's thuggery. Others have tried to quietly encourage Trump to embrace their hawkish position.

Republicans say they can live with some of Trump's unorthodox foreign policy positions. For instance, many of them privately agree with the president that the Iraq war was a mistake, even though they supported it at the time.

On Russia they are immovable. Republicans watched Bush's charm offensive fail and spent eight years criticizing Obama's "reset" policy. Significantly, they have seen no signs of a change in Putin's behavior since Trump assumed office.

"We're a little dismayed," Rep. Mike Simpson, R-Idaho, said. "If there are things we can work together on — fine. That doesn't mean you have to cozy up to [Putin] and suggest that Russia's activities haven't been detrimental."

Russia in recent days has provoked with the stationing of a spy ship off the Delaware coast, the buzzing nof U.S. warships with military fighter jets, and the deployment of cruise missiles in violation of a 1987 arms treaty.

Flynn, who has extensive ties to Moscow, resigned under pressure this week after it was discovered that he misled the administration about the content of his previous telephone conversations with the Russian ambassador.

These events, combined with Trump's stubborn bear hug of Putin and reports that officials on his 2016 campaign team were in regular contact with Moscow have moved some Republicans to push the administration to be more forthcoming about its relationship with Russia.

Even some Republicans who are concerned about the legality of wiretapping Flynn and leaking transcripts of his phone calls, are squeezing Trump to alter his Russia policy and to respond sternly to Putin's provocations.

In a move not often seen from Democrats under Obama, senior Republicans are pushing back. They support further Congressional investigations into Russia's meddling in the 2016 election that might implicate Trump.

Those same Republicans are now calling for those inquiries to examine the Flynn matter.

To tie the president's hands, House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell are pushing for new, codified sanctions on Russia. Trump has said he would be willing to relax existing sanctions under proper conditions.

Simpson said Republicans are holding the line on Russia because "it's what we believe."

Veteran Republican operatives explained that most Republicans in government today were raised in the era of President Ronald Reagan and the GOP's deep-seeded suspicion of the Kremlin.

Putin is a veteran of the old KGB Russian spy agency, and given his actions over the past decade and a half, both domestically and abroad, Republicans simply aren't willing to bend, as they have on other matters.

"Republicans know that Trump has a connection outside of Washington to disenfranchised voters … The deal breaker is Russia," a GOP political consultant said. "Most people across the board think that Trump needs to explain it further. If it's about containing China, most Americans would understand, but right now are confused."