(CNN) The virtually lockstep Republican defense of President Donald Trump so far during his impeachment trial marks a new milestone in the Senate's long-term evolution into a more partisan and regimented institution that demands unwavering party loyalty and punishes the freewheeling independence that characterized the great legislators through the body's history.

That transformation is grounded in a fundamental reordering of the political landscape: Over the past generation it has grown much more difficult for either party to win Senate seats in states that usually prefer the other party in presidential elections.

Today, the vast majority of senators from the President's party are elected by states that also voted for him -- increasing the pressure on them to stand with him -- while virtually all senators from the other party were sent by states that voted against the President, increasing the pressure to oppose him. Of the 53 Republican senators judging Trump, 51 were elected in states that backed him in the 2016 election.

These electoral pressures have contributed to remaking the Senate into the rigid, combative institution on display this week -- one in which the leadership exerts more control than in earlier generations, individual members are expected to display a level of party-line loyalty reminiscent of parliamentary systems in Europe and there is little leeway for the bipartisan deal-making that was the hallmark of great senators from Kentucky's Henry Clay in the 19th century to Kansas' Bob Dole and Massachusetts' Edward M. Kennedy in the late 20th.

"Everybody is pretty dug in and there's not a lot of space for forming coalitions that don't cleave in basically the same way across [multiple] issues," says Jeffrey Lewis, a professor of political science at UCLA who oversees an analytical project called Voteview that traces ideological and partisan patterns in every congressional roll call vote ever

It remains possible this week that the new revelation from former Trump national security adviser John Bolton -- alleging that the President directly told him he would not release military aid to Ukraine unless that country opened an investigation into his Democratic rival, former Vice President Joe Biden -- might prompt just enough Senate Republicans to defect from Trump and support calling Bolton and others as witnesses in the trial. But the fact that that vote still faces an uncertain prospect after such an explosive revelation -- one that directly undermines the central pillar of Trump's defense -- only underscores how unreservedly most Republican senators have locked arms to protect the President.

Votes for president and senators align

"It's a very unfortunate evolution in that the checks and balances that our Founding Fathers had so emphasized ... have been diminished if not completely eliminated," Tom Daschle, the former Senate Democratic leader from South Dakota, said in an interview. "There are very few checks and balances today in large measure because of the unswerving loyalty and commitment the Republican caucus has made to President Trump over and above their commitments to the role and the character of the Senate. It's been a diminution in stature, in authority ... in the ability of our constitutional order to provide the kind of real balance and oversight that is so critical to good government."

William Hoagland, the former longtime Republican staff director of the Senate Budget Committee, has been especially struck by the GOP deference to Trump as the President has eroded traditional congressional prerogatives, including redirecting funds for his border wall and systematically stonewalling subpoenas and other demands for information from the Democratic-controlled House during impeachment.

"The slap in the face of the institution, the equal branch of government, is amazing to me," says Hoagland, now a senior vice president at the Bipartisan Policy Center. "I don't know where they are. I don't understand what this guy holds over the Republicans that they don't understand they are independent United States senators that are representing the country and an institution separate and apart from the executive branch. They should be defending that institution, and they are not."

The electoral foundation of this new legislative order is the growing alignment between the way states vote for president and the senators they elect.

Long-term results from the University of Michigan's American National Election Studies , a post-election survey, capture the driving force in that trend: an increasing consistency in the choices voters make in their Senate and presidential votes. During the 1960s, the election studies' results show, 13% to 19% of voters backed presidential candidates of one party and Senate candidates from the other, according to figures provided by Emory University political scientist Alan Abramowitz. Through the 1970s and 1980s, consistently about 1 in 4 voters split their presidential and Senate votes between the parties.

But as the ideological contrasts between the parties have grown sharper over the past quarter century, that number has significantly declined: in each presidential election since 2000, only 10% to 14% of voters cast split-ticket ballots in Senate elections, according to the American National Election Studies findings.

Little incentive for bipartisanship

The powerful impact of this change has been to make it extremely difficult for either party to win Senate seats in states that usually support the other side in presidential elections. That's a big change from the middle decades of the 20th century. As I calculated in my 2007 book, "The Second Civil War," Republicans after 1972 held only half of the Senate seats in the states that voted two consecutive times for President Richard Nixon; after 1984, the GOP controlled just 55% of the Senate seats in the states that twice backed President Ronald Reagan.

But that figure has steadily increased since. After 1996, Democrats held two-thirds of the Senate seats in the states that President Bill Clinton carried both times; after 2004, Republicans held three-fourths of the Senate seats in the states that twice backed President George W. Bush.

Now, Republicans hold 92% (44 of 48) of the Senate seats in the 24 states that voted for both Mitt Romney in 2012 and Trump in 2016. Democrats, meanwhile, hold 38 of the 40 Senate seats (95%) in the 20 states that voted for President Barack Obama and Hillary Clinton in the past two elections. (Republicans hold seven of the remaining 12 seats in the six states that switched from Obama in 2012 to Trump in 2016).

Joe Manchin of West Virginia , Jon Tester of Montana , Kyrsten Sinema of Arizona and Doug Jones of Alabama are the only Democratic senators left in states that voted Republican for president in both 2012 and 2016, and Jones is facing a very difficult reelection next fall. Susan Collins of Maine and Cory Gardner of Colorado are the remaining Republican senators in the two-time Democratic states, and both also face difficult reelections, with Gardner a distinct underdog at this point.

Gary C. Jacobson, a University of California at San Diego political scientist who specializes in Congress, says the hardening alignment between presidential and Senate voting has made it vastly more difficult for senators to display the independence and receptivity to cross-party deal-making that once characterized the body.

"It changes things fundamentally," he says. "Voters at the state level vote very consistently for the same party for the House, Senate and president at much higher level than they did before. You can't really carve out a career [in Congress] with displays of independence that attract voters from the other side, or the remaining dwindling portion of people who are independent. So the career strategy has been to mobilize your base, and if that's the case you [tend to] win the states where your party's base is larger than the other party's base." With those stark electoral dynamics, he notes, senators and House members "are given very little incentive to even try" to show independence or reach bipartisan agreements.

Republican Party has been affected more

These changes have affected both parties. But most observers agree they have more fundamentally reconfigured the Republican Party, which operates with a coalition -- centered on conservative white voters -- that is more demographically and ideologically homogenous than the Democrats' is. That exposes Republican legislators who break from the party consensus to greater threats than Democrats face, from condemnation by the powerful conservative information system led by Fox News to primary challenges from the right and, more recently, to tweets of condemnation from Trump.

"I think it became perceived accurately as political suicide [for Republicans] to oppose him," Jacobson says. "Trump has shown that you can't do this gently and get away with it -- that he reacts very negatively to anything but abject praise, so all these people are walking a very fine line in trying to avoid not just criticizing him but saying anything that will attract his wrath, because they think he will be able to mobilize their base against him in their states or districts."

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The deferential Republican behavior during the Senate trial stands as both a monument to those immediate pressures and a new landmark in the extended remaking of Congress.

Particularly during the middle decades of the 20th century, both parties in Congress encompassed ramshackle coalitions of diverse views that compelled Democratic liberals and Republican conservatives to uneasily coexist with a large moderate block on each side. Political scientists in those years described Congress as operating with "four party politics" that included conservative southern Democrats, liberal Democrats from elsewhere, conservative Republicans from the heartland states and more moderate (and even liberal) GOP legislators from states along the coasts.

Accomplishing anything in Congress during this period -- roughly from the late 1930s through the early 1980s -- required complex bargaining among these factions. It was the ability to wrangle episodic, idiosyncratic coalitions across party lines that made senators such as Lyndon Johnson of Texas, Everett Dirksen of Illinois, Howard Baker of Tennessee, Dole, Kennedy and later Arizona's John McCain such powerful forces.

"There used to be a block of relatively unattached people [in Congress] who you would say were persuadable one way or the other by the facts," notes Lewis.

But at least since the early 1980s, the overriding trend line in both congressional chambers has been increasing ideological cohesion within the parties and widening ideological distance between them, a dynamic that has produced more party-line voting and less cross-party deal-making.

The full force of this change was first felt in the House, especially following the new Republican majority after 1994 led by then-House Speaker Newt Gingrich of Georgia changed several internal rules -- such as the manner in which committee chairmanships were selected -- to maximize the leadership's leverage to command loyalty from rank and file members. The new GOP majority centralized decision-making in the leadership and exerted enormous pressure on members to vote in unison behind its choices. When Democrats regained the House majority from 2006 to 2010, and again in 2018, they largely upheld the shift toward centralizing authority in the leadership.

A more top-down style

Initially, this wave did not wash as heavily over the Senate. But over time, the Senate too, has grown more to resemble a parliamentary institution, in which the leadership makes the key decisions and there is little tolerance for dissent from those decisions by individual members.

"Over the course of my time, but especially in the later years, politics in the Senate became much more of a top-down-driven process ... where leadership wielding a few chits -- including the ability to raise money -- became more powerful than even the most powerful chairmen of the most significant committees," says Jim Manley, a former communications adviser to both Kennedy and former Democratic leader Harry Reid of Nevada.

One way to measure this change in the Senate is to track its response to the principal presidential scandals of the past half century. During Watergate, Nixon resigned in August 1974 only after a delegation of senior GOP senators, including iconic conservative Barry Goldwater of Arizona, bluntly told him in the White House that his support in the chamber had crumbled. By the time of the Iran-Contra scandal in 1986 and 1987, no serious move developed to impeach or remove Reagan. But Republicans participated in a bipartisan bicameral investigation and though there was greater partisan acrimony, the Senate hired a bipartisan staff for its component of the investigation and three Republican senators signed the final report , which sharply criticized Reagan's actions.

After the Republican-controlled House in late 1998 impeached Bill Clinton over allegations emerging from his affair with a White House intern, Democrats also maintained some distance from the White House during his Senate trial in 1999. While no Senate Democrat voted to remove Clinton from office, all but one voted to bring to the floor a resolution to censure him for his behavior sponsored by Democratic Sen. Dianne Feinstein of California. (The resolution failed on a procedural vote.) And many Senate Democrats sharply criticized Clinton for his behavior, none more cuttingly than Daschle, the party's leader, who said at the time, "I have never been, nor ever expect to be, so bitterly disappointed again."

The censure resolution, Daschle told me this week, "was a desire to find some way to respond to what we clearly saw was an infraction ... that may have fallen short of high crimes and misdemeanors but certainly couldn't be ignored."

Compared with even that middle ground by Senate Democrats in the Clinton case, almost all Republican senators have been far more reluctant to hint at any objection to Trump's behavior.

"I don't think there was any doubt that there was a dramatic difference in our approach and our response to the circumstances we faced versus what we see from them today," Daschle says.

Hoagland, the longtime senior GOP Senate aide, agrees. He finds it especially striking that it is Senate Majority Leader Mitch McConnell who is leading the effort to align the party behind an indivisible defense of Trump, even at the price of weakening the Senate's institutional powers, such as the ability to compel testimony and demand documents from the executive branch. Powerful Senate majority leaders of earlier generations, such as Johnson or Robert Byrd of West Virginia, would be stunned at the Kentucky Republican's deference, Hoagland says.

"The one that is most discouraging to me is Leader McConnell," he said. "The Bob Byrds of the world have to be turning over in their grave, the way this is being handled by the leader."