A year and a half into Trump's presidency, Republicans are learning now more than ever that the GOP is Trump's party and that's leading to some tough choices.

From trade policy to primary endorsements, the President's positions -- even if they challenge long-established Republican orthodoxy-- are redefining what it means to be a part of the GOP.

After a stunning loss Tuesday night, South Carolina Republican Rep. Mark Sanford became the latest casualty in the fight for the GOP's future . A member of the conservative House Freedom Caucus, Sanford exemplified the conservative, less-government philosophy -- that was once the cornerstone of the party. But, in the eyes of voters, Sanford's free-wielding jabs at Trump, his candid comments about the President's leadership style, overshadowed any allegiance he had to conservative principles.

"Mark Sanford is a true conservative. He's one of these guys that when he talks about what I believe our ideology is, he believes it. And he can speak to it intelligently," said retiring Rep. Tom Rooney, a Republican from Florida. "The fact that somebody who's a true conservative can't win a primary in South Carolina, a member of the Freedom Caucus, just goes to show what's more important. What's more important obviously is loyalty to Trump."

It's the fundamental choice that each and every Republican member of Congress has to make for themselves. A party that once celebrated America's influence abroad, must accept a President who wants the US to turn inward. A party that once believed in free trade must rationalize a President who imposes tariffs on the country's closest allies and employs aides who attack the Canadian prime minister. There's the rhetoric, ad-hoc decisions and shifting positions on even as something as fundamental to the party as the Second Amendment.

On Wednesday, Tennessee Republican Sen. Bob Corker -- unencumbered by the leash of re-election or fear of a President who could with one tweet alter his political career in Washington forever -- stopped for the cameras and diagnosed what he thought was happening with his party.

"We're in a strange place. It's becoming a cultish thing, isn't it?" the Tennessee Republican told reporters. "It's not a good place for any party to have a cult-like situation as it relates to a President that happens to be purportedly of the same party."

The latest hostage

Corker's comments came just a day after he delivered an animated speech on the Senate floor in which the Tennessee Republican accused GOP leaders of blocking his amendment on the National Defense Authorization Act because they were afraid to defy Trump in an election year. Corker's amendment would have rolled back the President's power to enact tariffs on the grounds of national security and had already attracted Trump's ire.

"We might poke the bear!" Corker said on the floor . "My gosh, if the President gets upset with us we might not be in the majority," he said referring to sentiments he often hears from colleagues.

Asked about Corker's comments about the GOP being cult-like, Sen. Thom Tillis said, "That's the words of somebody who's frustrated because he didn't get an amendment passed."

The National Defense Authorization Act, a bill that has broad bipartisan support, has become the latest hostage in the showdown between the Trump administration and Republicans in Congress. After striking a deal that would save Chinese Telecom company ZTE, Republicans and Democrats banded together to add an amendment to the NDAA that would essentially undo the Trump administration's actions. But, the administration vowed to fight the amendment and a handful of Republican senators rushed to try and extract the amendment from the bill.

Sen. David Perdue, a Republican from Georgia, went to the Senate floor Wednesday night to get the amendment out of the NDAA, and is among those who believe Republicans need to be careful defying Trump when the world is watching.

"We all need to be very careful about our statements because they are being listened to very carefully by people around the world who we are in the middle of negotiations with," Perdue said.

'We try to stay focused on the agenda'

The trade off for Republicans in leadership is one where they need Trump. They want the President -- the first Republican since President George W. Bush -- to enact their agenda, to sign bills to overhaul the tax code and repeal Obama-era regulations.

South Dakota Republican Sen. John Thune said maintaining a relationship with the President -- even if it can be challenging -- is required to make the GOP's priorities, law.

"I know that the Republican leadership views the relationship with the President as one where we got to be able to get things done together and so we try to stay focused on the agenda and try not to focus on the daily distractions that seem to come around here on almost an hourly basis," Thune said.

Thune added that there were more tough discussions than meets the public eye.

"I think there's a lot of pushback that goes on that perhaps sometimes maybe you don't see," Thune said. "Offline conversations with the administration, the President, with his team, on some of these specific issues. But again, I think it's probably not in anybody's best interest for trying to get an agenda done to have divisions within the ranks."

For now, the loudest voices in the party -- the ones who say they feel free to speak out against Trump -- are members who are leaving on their own terms. Senators like Corker and Arizona Sen. Jeff Flake, another retiring Republican.

"It's always 'let's wait and get through the midterms and then we'll rein the President in' or 'then we'll assert our constitutional prerogatives,' but we've got to be all together now," Flake said.

The Republicans who can, speak out. For the ones who disagree but care about their next election, the message appears to be: Excuse Trump's behavior, bemoan his tweets, stay quiet or retreat, bending in ways both large and small to appease a President most never expected to set foot in the Oval Office.

"If you value re-election above all else, then find a way to accommodate the President," Flake said. "I don't know what other message you can draw from what's happened. This is the President's Party."