There has long been a strain of nationalism that has sought to root itself in the Christian faith. Before the Worldwide Church of God literally repented and moved from cult to mainstream denomination, Herbert W. Armstrong endorsed a hardcore Anglo/American Israelism, in which God was deemed to have transferred the ancient promises to the Jewish people to the white Protestant (ideally mostly British) followers of Armstrong. But there have been milder strains that simply tried to view America as a unique country with special purpose in the world, a "city set on a hill" as one Puritan divine once preached, citing exclusive imagery that applied to ancient Israel in its original biblical context.
Christian Nationalism is the belief that "the American nation is defined by Christianity, and that the government should take active steps to keep it that way" (from Christianity Today). This has often been expanded to include the "Judeo-Christian" morals and values that mid-20th Century politicians fused to American "civic religion", seeking common cause with Jewish people post-WWII against "godless Communism". But it has been by and large a predominantly Christian reference point, essentially fusing (and confusing) aspects of Christian faith with American patriotic symbols and politics.
Likewise, a virulent type of Christian Nationalism has festered and grown more prominent in recent decades that endorses a racist ideology, viewing black and brown-skinned people (and now people of Asian decent, post-COVID) as essetnially other. Additionally, the post-Civil Rights era shift of former Dixiecrats into the GOP has galvanized racist rhetoric and policy pursuits that off go ignored or are even "baptized" in Christian language. For example, when Mississippi made it illegal to even assist an undocumented immigrant in need, the "Christian" politicos and pundits pulled St Paul out of context to talk about obeying civil authorities, despite the fact that Jesus' teaching uniformly admonishes us to help those in need...even our enemies.
So Christian Nationalism, as it now stands, is a potent, syncretistic mix of patriotic fervor, racist fearmongering, and political maneuvering. Crazily, the complaint that Christians are a persecuted minority, despite its hegemonic influence for centuries in the U.S., is being touted, as "non-Christian" minorities are starting to outpace the "Christian" white majority demographically. No matter that the largest numbers of minorities are Catholic Latinos and Evangelical Blacks...who are considered within the Christian fold by most metrics except the most bigoted ones.
Aside from the racist undertones that motivate way too many expressions of Christian Nationalism, the problems theologically are manifold. First, it blurs the line between party politics and religious faith. It's one thing to have one's faith inform your moral conscience, which in turn informs your beliefs about best policy. But we must recognize that people can hold a variety of political beliefs while still being faithful to the Gospel. A British evangelical who votes Labor or Socialist would almost never vote GOP if they emigrated to the States. Likewise, a number of people who are Trump supporters voted for him "pinching their nose" in spite of notorious character flaws in the hopes that with a unified Congress and White House certain moral issues (religious liberty protections, limiting abortion, etc.) might be made public policy. It's deeply unlikely in a pluralistic nation like ours that some of these issues will ever be settled to certain Christians' satisfaction, simply because we must make room for those who do not hold the same values or religious bases for them.
Second, it mishandles the Bible. There is no way to import the laws or promises of ancient Israel to an American context because 1) we're NOT Israel, and 2) America is not the Church. Many of the Old Testament commands and promises are no longer applicable because they were limited to the people who claimed Abraham, Isaac, and Jacob as ancestors, and even then they applied only while there was a monarchy or a temple. The ones that do "carry over" to the Church are specifically mentioned in the New Testament, in short, the "moral" ones (like the Ten Commandments). But even then, we cannot confuse the promises God gives to his people in the Church with promises to the nation. Does God care about America? Certainly. But American is not the centerpiece of his plan. Jesus is, and then the Church as they are part of his body. So American exceptionalism is not part of the biblical record. Indeed, I wouldn't be surprised if we're at the top of his list to judge for all the hypocrisy we've exhibited in advocating freedom and democracy while propping up dictators and allowing systems of public policy and justice go unchallenged that offer darker hued Americans with unfair and unequal treatment.
Finally, it creates an atmosphere of "us vs. them". The way politicians pander to people's most base impulses should be criminal. Our neighbors are not the enemy just because they look different, don't worship the way we do (or not at all), or even vote differently than we do. They are only enemies if they infringe our basic rights (life, limb, liberty to pursue God as we are convinced,, etc.). And even then, Jesus is pretty clear about turning the other cheek and doing right by them anyway.
So is it possible to be patriotic and be a Christian? Certainly, if you recognize duty to country as subordinate to duty to God. You can love America and strive to make it better and be glad for the freedoms you enjoy. You can also be deeply concerned for its soul and disappointed in how it's fallen short of its own stated ideals and not be branded traitor. You can even choose to be apolitical by right, like historically many Anabaptist groups have done. But let's not forget being American doesn't mean you're Christian. Nor does being Christian mean you need to belong to a particular party or have a the same view as other Christians across the board on whatever hot button issue of the moment is driving public discourse. There's room for nuance and disagreement, and you won't be considered a traitor. Hopefully, you go a church that won't deem you anathema for simply having a different mind on political issues (since most are outside the basics of the faith) than many or most of the rest who attend. And hopefully, you offer the same charity to those with home you disagree on particular issues.