Both President Trump and British Prime Minister Boris Johnson are waging the political battles of their lives. Already in campaign mode ahead of the 2020 presidential election, Trump now finds himself locked in a damaging, spiraling impeachment inquiry into his possible abuses of power. Johnson, meanwhile, entered office this summer determined to yank his country out of the European Union by hook or by crook. But he has been thwarted by lawmakers, as well as Britain’s Supreme Court — making a mockery of his vows of a “do or die” Brexit by the end of this month.

Still, the two leaders on both sides of the pond are bent on subjecting their countries to a bitter, tribal fight.

Every day, as more damning evidence comes out about the president’s attempt to pressure the Ukrainian government into investigating the son of a domestic political rival, Trump lashes out at his supposedly illegitimate political opponents and the “corrupt” media reporting on the imbroglio. He referred last week to a small but influential coterie of anti-Trump Republican intelligentsia as “human scum.” And he punctuates his incessant narrative of victimization with frequent tweeted reminders of the near-lockstep support he commands from within his Republican base.

Johnson is now readying for a Dec. 12 general election, a contest he probably wanted. Unlike Britain’s 2016 referendum result, the majority of the members of the current House of Commons are probably not in favor of Brexit, and even more oppose the “no-deal” crash out of the European Union that Johnson has said he’s willing to execute. The prime minister, though, has tied his political fortunes to a hard-line, pro-Brexit base of support and will be hoping their fury at the latest delay — Britain got another formal extension from the European Union to conclude their divorce deal by the end of January — will translate into a more effective parliamentary mandate before Christmas.

In the meantime, he has borrowed from Trump’s populist demagoguery, casting Parliament as the enemy of the people and his opponents as guilty of “surrender” and “betrayal.” He rejected criticism of his inflammatory language as “humbug.”

But as the two leaders slug it out with their opponents, the political parties they lead are feeling the heat. Trump has viciously turned against any former lieutenant, top administration official or Republican politician who has publicly contradicted him or criticized his actions. His proxies, including broadcasters at right-wing Fox News, have smeared career officials who delivered potentially damning testimony about Trump’s conduct. This week, that included Lt. Col. Alexander Vindman, a Soviet refugee turned American war hero who served as a Ukraine expert within Trump’s White House.

In America’s febrile political moment, noted Post columnist Greg Sargent, for the president and his allies, “there is no greater expression of disloyalty to country than a display of disloyalty to Trump.”

A number of senior Republican politicians condemned the “shameful” pro-Trump attacks on Vindman’s patriotism. Many in private express a deeper disquiet with the course of events, especially the reckoning that may follow a possible House impeachment of the president.

“In hushed conversations over the past week, GOP senators lamented that the fast-expanding probe is fraying their party, which remains completely in Trump’s grip,” reported The Post’s Robert Costa and Philip Rucker. “They voiced exasperation at the expectation that they defend the president against the troublesome picture that has been painted, with neither convincing arguments from the White House nor confidence that something worse won’t soon be discovered.”

But there’s little indication for now that any of these senators will risk Trumpist ire and back his constitutional removal. “They’ve decided that they’re going to take it all grudgingly — and privately, perhaps, in disgust — but they’re not going to give up the farm,” Al Cardenas, former chairman of the American Conservative Union, told The Post.

In Britain, Johnson has already tested the unity of his ruling Conservatives. His tight embrace of a Brexit at all costs has been boisterously received by the country’s jingoistic tabloid press, and it may persuade pro-Leave voters otherwise eyeing the Brexit Party of populist gadfly Nigel Farage to remain in the Tory fold.

But “the approach has come at the expense of a full-scale rupture among the Conservatives,” my colleague Griff Witte wrote earlier this year. “The most successful party in British political history has been divided for decades over the country’s membership in the E.U., with room both for fervent supporters and opponents.”

A series of anti-Brexit or Brexit-skeptic Conservative lawmakers, including Johnson’s own brother, either quit the party or stepped down from senior-level posts. Some analysts now see the Conservative Party as a faction that stands for little beyond Johnson’s quest for power — something that remains firmly in his reach as the opposition Labour Party struggles with its own divisions under the stewardship of leftist Jeremy Corbyn.

“Surviving Tory moderates … kid themselves that the party is still theirs, as if it hadn’t swelled with Brexit fanatics with no interest in governing,” British sociologist William Davies wrote earlier this month. “These men are opportunists, for whom nationalist fervor and chaos provide an opening to seize power. And a party in the grip of collective mania is more vulnerable to such machinations than one with a coherent ideology.”

A similar diagnosis could be offered for Republicans who have sought to normalize Trump’s behavior. But the polarization that now defines politics in both the United States and Britain was weaponized by Trump’s and Johnson’s parties long before either came to power. And its grim effects are now entirely in view.

“We’re perilously close to the point at which there may no longer be a national consensus that there’s anything constitutionally problematic about using governmental powers to advance one’s own pecuniary and electoral interests,” legal scholars Marty Lederman and Benjamin Wittes wrote in the Atlantic, gesturing to the climate of conspiracy theory and denial propagated by Trump’s backers.

“Electoral uncertainty should not be cause for alarm in a mature democracy, and true democrats do not shy away from a polling booth,” wrote Guardian columnist Rafael Behr. “Still, it is hard to shake the fear that British politics will be even angrier and more divided after an election campaign than it is now.”