Washington (CNN) Republican hopes of blocking former national security adviser John Bolton's impeachment testimony highlight the Trump-era GOP's defining characteristic: its refusal to defy the President under almost any circumstances.

More significantly, decades of American political realignment have tightened the bonds holding Republicans together in any high-stakes fight with Democrats. The ongoing diversification of American society further unites an overwhelmingly white GOP around a shared fear of impending doom.

Together, those changes lend weight and rigidity to Republican partisanship that did not exist that last time a GOP chief executive faced impeachment. The modest barrier that once separated Republicans from Democrats has become a dense, multi-layered wall reinforced by ideology, race, education and religion as well as party identification.

That helps explain why so many Republican senators have cast aside consistency, logic and unrebutted evidence to shield their president from impeachment charges. Senate Majority Leader Mitch McConnell aims for a Friday Senate vote in which Republicans would refuse even to hear new testimony about Trump's culpability before summarily acquitting him.

American partisanship has not always proven so powerful. Two decades ago, five House Democrats voted to impeach President Bill Clinton on charges stemming from his adulterous affair with a White House intern. Senate Democrats stood united to acquit him -- but only after condemning wrongdoing that Clinton himself acknowledged.

And two decades before that, outright defections by Republicans in both the House and Senate sealed the outcome of the Watergate scandal and forced President Richard Nixon's resignation in 1974. That result is virtually unimaginable in 21st century Washington.

Nixon-era voters wore their Republican and Democratic labels far more lightly than voters today. Identification with their parties, both ideologically scrambled under alignments that had persisted since the Civil War, represented only one element of their political identity and behavior.

White Southerners in Congress identified with Nixon's conservatism -- but generally belonged to the opposition party. In 1974, Democrats still held 17 of 26 Senate seats in the 13 states of the old Confederacy.

Many Northern liberals shared the President's GOP affiliation but disdained his politics. In the 11 states of the Northeast, Republicans held 13 of 22 Senate seats.

Republicans such as then-Reps. William Cohen of Maine and Hamilton Fish of New York broke with Nixon to join Democratic leaders in backing impeachment in the House. GOP Sens. Edward Brooke of Massachusetts and Lowell Weicker of Connecticut called for the President's resignation long before Nixon took that step in August 1974.

Partisan allegiances had already begun shifting amid the upheavals of the 1960s. After President Lyndon Johnson signed landmark civil rights legislation -- with support from a Senate Republican leader from Illinois -- white voters drifted toward the GOP while African Americans cemented their alliance with Democrats.

By now, those shifts have increased ideological uniformity and consistency within each party. According to 2016 election exit polls , 8-in-10 liberals and 9-in-10 Democrats backed Hillary Clinton. Comparable proportions of conservatives and Republicans backed Trump. Southern states have aligned their conservatism with GOP partisanship, Northern states the reverse.

Polarization by race reinforces those cleavages. While 9-in-10 African Americans backed Clinton, nearly 6-in-10 whites backed Trump.

The Republican tilt is even more lopsided among white evangelical Christians and those without college degrees. Both feel threatened by demographic and cultural changes that have tripled the size of the non-white electorate, and cut in half the share of Americans who are white Christians, over the last half-century.

Democrats have won the popular vote in six of the last presidential elections. Reflecting Republican foreboding, Trump's Attorney General William Barr recently warned that "secularists, and their allies among the 'progressives' " have sustained "an unremitting assault on religion and traditional values" in a campaign of "organized destruction."

That view of the stakes generates immense pressure against defection within the GOP. In the House, with Trump denying any wrongdoing on Ukraine whatsoever, not a single Republican voted to impeach him.

In the Senate, no Republican has called for the President's resignation or signaled intent to vote for his conviction. Democrats need support from four of 53 Republicans just to subpoena Bolton's testimony, which reportedly promises to affirm the Democratic allegations. The combined efforts of McConnell and the White House may block even that.

The decisive vote on Bolton's testimony may lie with retiring Republican Sen. Lamar Alexander of Tennessee, once an aide in Nixon's White House. His own political emergence a few years after Nixon resigned lends poignancy to his choice against hearing witnesses, which he announced late Thursday night

After Alexander won the Tennessee governorship in 1978, the outgoing Democratic governor began accepting payments in exchange for pardoning convicted criminals. Crossing party lines, other top Democratic officials arranged to halt that corruption by swearing the newly elected Republican into office early.