(CNN) Nine years ago, then-South Carolina Republican Gov. Mark Sanford's political career seemed to disappear along the Appalachian Trail. Sanford was able to resurrect his political life with a successful bid for Congress from South Carolina's First District, even after it was discovered that his trail trip was actually an extramarital affair in Argentina.

On Tuesday, Sanford's career may go down the tubes again. Unlike nine years ago, when it took days to figure out that Sanford wasn't actually on the trail, it doesn't take days to determine why Sanford is in trouble in 2018.

Sanford is learning a lesson that should be pretty clear by now: the Republican Party belongs to President Donald Trump , and most Republicans who go against him are endangering their political lives.

Sanford has been vocal about the times he has disagreed with Trump. For instance, he supported Trump in 2016, but called on him to release his tax returns. Sanford said Trump's violent rhetoric was "a problem" in 2017.

And when it came to actual legislation, only four out of 244 Republican House members have voted with the President less often than Sanford, according to a FiveThirtyEight analysis.

The disagreements between Sanford and Trump have been a focal point for Sanford's primary opponent Katie Arrington, and it seems to be working for her. Sanford's been spending money on television advertisements for the first time in five years . A recent survey from a local pollster (that does not call cell phones and does not meet CNN's standards) has Sanford leading Arrington, but the lead was within the margin of error.

Sanford even gave out his own personal cell phone number for voters to call him.

In other words, Sanford could win Tuesday, but it's also quite possible that he could lose.

Sanford is hardly the first Republican member of Congress to have primary problems because of how he dealt with Trump. Republican Sens. Jeff Flake of Arizona and Bob Corker of Tennessee , both critics of the President, dropped out of their respective primaries, in which, they were seen as insufficiently pro-Trump. Just last week, Alabama Republican Rep. Martha Roby was forced into a primary runoff, in large part because of blow back she is receiving for withdrawing her endorsement of Trump during the 2016 campaign.

The problem that Trump's Republican adversaries are learning is that it's not 2016 anymore. Even as Trump was winning the presidential election, his net favorability (favorable-unfavorable) rating among Republicans in an average of polls was just +50 points. That's far short of the maximum +100 point rating. You could, in other words, oppose or critique Trump as a Republican (as Sanford did in 2016) and not get chased out of the party.

Today, it's totally different. Trump's net favorability among Republicans is up to +71 percentage points on average. There are very few Republicans who disagree with Trump. Trump's approval rating (a slightly different measure than favorable rating) stands at 87% in the latest Gallup poll . To put that in perspective, only two other presidents since 1950 have had higher approval ratings among their own party heading into a potential presidential primary.

The idea of a Republican opposing George W. Bush or Ronald Reagan (who had approval ratings among Republicans most similar to Trump) seems nutty on its face. There's a reason why both of them marched to renomination. The same idea seems to hold true for Trump. If there's any record of a candidate opposing Trump, it could be a problem for them in 2018.

That's a lesson Sanford may learn all too well on Tuesday.