The American system of government, and in particular its constitutional furniture (Congress, the Supreme Court, the Presidency), is constitutive of national politics as much as it is representative. In other words, the national polity is periodically recreated through the democratic process. This is why US politics is highly prescriptive, obsessed with procedural manoeuvres, and treated as a spectator sport by the media. The spectacle of the US Presidential campaign is particularly didactic in that it takes candidates and voters on a journey from ward-level caucuses through state primaries and inter-state conventions to a nationwide election and the abstractions of national interest. The trajectory serves to marginalise interests that don't easily fit into this structure. For example, race tends to enjoy a brief salience in the South Carolina primary but is usually kept to the undercard elsewhere.
It also produces the paradox of a nation famed for its parochialism obsessing over foreign policy issues, even if they are addressed in cartoonish terms. A gradual process of refinement and moderation, heavily mediated in the latter stages by Washington elites and the traditional sages of foreign affairs (Henry Kissinger will shortly pop up), produces a final consensus that is pretty much "more of the same" with the promise of better behaviour. It also tends to bias towards rhetorical moralism and the global responsibilities of "the indispensable nation", which diverts attention from more obvious economic self-interest, so Russia will usually figure more than South America. This is why American Presidential elections tend to have more foreign policy substance than you'd expect, and why domestic debate often dances around proxies for race: crime, welfare, the "middle class" (i.e. white working class).
There has been plenty of commentary since Obama's election in 2008 about the demographic tilt in the USA and how this is likely to influence future elections. The conventional wisdom holds that the Democrats are a shoo-in because of high levels of support among growing minority populations, as well as the socially liberal but economically-stressed young; while the Republicans will struggle to forge a viable electoral alliance between Hispanic voters across the sun-belt and mad-as-hell seniors across the Midwest. This assessment assumes that blacks will always favour the Democrats and that there is a latent conservative majority among Hispanics (and not just Cuban exiles). This is a bit like the idea that the Scots will always vote Labour and that Asians have yet to wake up to the attractions of the Tories. It's true until it's clearly not.
American conservatism has three pillars: nationalism, capitalism and religious morality. Until the 1950s, nationalism was heavily conditioned by the US's origins as a racist and genocidal state and its belief in the "manifest destiny" of whites. The Civil War may have led to the emancipation of slaves, but it also standardised racial discrimination as a principle of social organisation, as blacks were transformed from chattels to second-class citizens. Race-inflected nationalism in the late 19th century served to advance capitalist interests in the near-abroad, e.g. in the war against Spain, but it also served a domestic purpose in dividing the working class between natives and immigrants, with the latter being defined by culture (e.g. Catholic Irish and Italians) as much as ethnicity (e.g. Chinese).
The value of race as an organising principle weakened as immigrant groups assimilated, blacks moved to the northern industrial cities, and the Great Depression crossed racial and cultural lines. With uncertainty over what constituted a popular nationalism - no longer WASP, let alone centred on small farmers and craftsmen - American identity started to define itself through populist and isolationist opposition: against trusts and robber barons; foreign entanglements and European imperialism; and the red menace of the Soviet Union and China. Anticommunism became central to US politics for two reasons. First, it provided a form of ideological nationalism that marginalised race and class (though this ironically created an environment in which civil rights could be advocated as pro-American). Second, it provided support for the elite pivot from isolationism to internationalism in the 1940s.
After 1989 and the sudden redundancy of anticommunism, US nationalism was reworked by neo-conservatives as the proactive assertion of US economic interests abroad. For all the guff about exporting freedom, this was clearly a return to "realist" geopolitics. This not only maintained America's internationalist focus, but self-consciously connected with the "imperial" lineage of the late 19th century before the turn to isolationism. The problem was that this policy's triumphalism and delusion ("we make our own reality") demanded permanent success. When it hit the buffers in Iraq, the policy framework fell apart. Some conservatives sought to fill the vacuum this created with Islamophobia, but that hasn't provided anywhere near as compelling a threat to America as international communism once did, and the general population remain sceptical that terrorism is sufficient justification for sending American troops (as opposed to drones) overseas.
While white resentment at minority advances remained a strong undercurrent during the conservative revival of the 70s and 80s, it was never strong enough to turn nationalism back towards a nativist or isolationist stance. This was partly because of the Reagan era's aggressive internationalism, but also because conservative strategists deliberately channelled the resentment into an attack on a welfare state that was seen as a proxy for "those people", i.e. blacks and other minorities. The recent shift towards greater support for welfare in the US reflects demography as much as the economy. While the increase in income inequality and the poor prospects for "millennials" grab the headlines, the sea-change is that whites are due to become a minority of the US population by 2043, so "those people" will increasingly be funding the welfare state of tomorrow. The growing intolerance of police violence and guns are signs of this social change: both the growing confidence of minorities and the growing rejection of institutional racism in their name by whites.
The trope of "war" in domestic American politics is usually a sign of elite interests being pursued through divide-and-rule. Cold War anticommunism helped hobble the American labour movement at a time when it was economically powerful, and also provided a means of hampering support for the extension of civil rights. The "War on Drugs", and the subsequent growth of the carceral state, like the rhetorical attack on "welfare queens", was a cynical manoeuvre to divert the resentment of whites, faced by destabilising social and economic changes, towards blacks; a manoeuvre embraced by the Democrats as much as the Republicans. The "War on Terror" has served to create a new "other" (i.e. Muslims) at a time when black and Hispanic voters have become too significant as constituencies to either ignore or alienate. But America is war-weary, and the paranoid style promoted latterly by Fox News is facing diminishing political returns as it becomes ever more internecine and absurd.
Barack Obama's presidency may have been marked at the beginning by the antics of the Tea Party, but it has been marked at the end by Black Lives Matter. The significance of Beyoncé delivering a Black Power tribute at the Super Bowl is that a TV network dependent on advertisers seeking a broad audience did not bat an eyelid. The problem faced by the political establishment is that America is running out of enemies. This is not because it is drawing in its horns internationally, but because it cannot identify a convincing, common threat that unites an increasingly diverse domestic audience. Global jihad doesn't measure up as an existential danger, old dichotomies like white versus black no longer work for enough people, while the self-identifying middle class is increasingly open to single-payer healthcare, the legalisation of cannabis and more progressive taxation.
Donald Trump is the logical result of the "plot against America" turn of Republican politics. Where other candidates are reduced to threatening to carpet-bomb far away countries where evil villains may be hiding, Trump happily singles out Mexicans and Muslims on US soil. His popularity reflects the rejection by the Republican base of the conventional wisdom that the party must reach out, but there is little evidence that circling the wagons will be electorally successful. In contrast, there is growing evidence that the demographic dividend promised for the Democrats is more likely to be realised by a social democrat like Bernie Sanders than by a neoliberal like Hillary Clinton for whom Wall Street funding, race and gender are all instrumental. As the field thins, it is the "1%" who are increasingly being cast as public enemy number one, almost by default.