Several months ago I was asked what advice I would give to the Trump campaign.

I said, only half joking, that he had better pick a vice presidential candidate the establishment hates more than it hates him. That would be his only insurance against impeachment. Those drums have already begun to beat, be it ever so subtly.

Is anyone surprised how quickly the establishment that Donald Trump campaigned against has announced opposition to much of his policy agenda? No. But few understand that the passionate opposition includes a willingness to impeach and remove President Trump if he does not come to heel on his America First goals.

Ferocious opposition to Trump from the left was expected and thus surprises nobody. From the comical demands for vote recounts to street protests by roving bands of leftist hate-mongers and condescending satire on late-night television, hysterical leftist opposition to Trump is now part of the cultural landscape.

But those are amusing sideshows to the main event, the Republican establishment’s intransigent opposition to key pillars of the Republican president’s agenda.

Republican leaders in Congress are already sending Trump a subtle but clear warning: accept our business-as-usual Chamber of Commerce agenda or we will join Democrats to impeach you.

If you think talk of impeachment is insane when the man has not even been sworn into office yet, you have not been paying attention. Impeachment has been the goal of Democrats since the day after Trump won the election, and the Republican establishment will use the veiled threat as leverage to win concession after concession from the Trump White House.

What are the key policy differences that motivate congressional opposition to the Trump agenda? There are at least four Trump campaign promises which, if not dropped or severely compromised, could generate Republican support for impeachment: Trump’s Supreme Court appointments, abandoning the Trans Pacific Partnership, radical rollback of Obama regulatory projects, and real enforcement of our nation’s immigration laws.

On regulatory rollback, Congress can legitimately insist on negotiating the details with Trump. But on the other three, immigration, the TPP, and Supreme Court nominees, Trump’s campaign promises were so specific — and so popular — that he need not accept congressional foot-dragging.

Yet, while the President-elect ‘s transition teams at the EPA, State Department and Education Department are busy mapping ambitious changes in direction, Congress’s Republican leadership is busy doubling down on dissonance and disloyalty.

Senate Republican leader Mitch McConnell announced this week he will oppose Trump’s tax reforms.

Senator Lindsey Graham is joining Democrats in sponsoring new legislation to protect the “Dreamers” from deportation after their unlawfully granted legal status and work permits expire.

Senator Susan Collins will oppose any restrictions on Muslim refugees, no matter how weak and inadequate the vetting to weed out jihadists.

Senator Lamar Alexander aims to protect major parts of Obamacare, despite five years of voluminous Republican promises to “repeal and replace” it if they ever had the power to do so.

And then, on the House side, we have the naysayer-in-chief, Speaker Paul Ryan, who refused to campaign with Donald Trump in Wisconsin, and who has vowed to obstruct Trump’s most important and most popular campaign promise — an end to open borders and vigorous immigration law enforcement.

It is no exaggeration to say that Trump’s success or failure in overcoming the opposition to immigration enforcement will determine the success or failure of his presidency. If he cannot deliver on his most prominent and most popular campaign promise, nothing else will matter very much.

So, the bad news for President Trump is this: If he keeps faith with his campaign promises on immigration, for example to limit Muslim immigration from terrorism afflicted regions, which is within his legitimate constitutional powers as President, he will risk impeachment. However, his congressional critics will face one enormous hurdle in bringing impeachment charges related to immigration enforcement: about 90 percent of what Trump plans to do is within current law and would require no new legislation in Congress. Obama disregarded immigration laws he did not like, so all Trump has to do is enforce those laws.

Now, if you think talk of impeachment is ridiculous because Republicans control Congress, you are underestimating the depth of Establishment Republican support for open borders.

The first effort in the 21st century at a general amnesty for all 20 million illegal aliens came in January 2005 from newly re-elected President George Bush. The “Gang of Eight” amnesty bill passed by the US Senate in 2013 did not have the support of the majority of Republican senators, and now they are faced with a Republican president pledged to the exact opposite agenda, immigration enforcement. And yet, do not doubt the establishment will sacrifice a Republican president to protect the globalist, open borders status quo.

The leader and spokesman for that establishment open borders agenda is not some obscure backbencher, it is the Republican Speaker of the House. Because the Speaker controls the rules and the legislative calendar, if he chooses to play hardball against Trump on immigration he can block any of Trump’s other policy initiatives until Trump abandons his immigration enforcement goals.

What all this points to is a bloody civil war within the Republican Party fought on the battlefield of congressional committee votes.

Donald Trump won a electoral mandate to change direction and put American interests first, beginning with border security. If the congressional Republican establishment chooses to block the implementation of that electoral mandate, it would destroy not only Trump’s agenda, it would destroy the Republican Party.