(CNN) The lockstep congressional Republican defense of President Donald Trump through every stage of the Ukraine inquiry raises troubling questions about whether the impeachment process can still provide an effective check on a President determined to abuse his authority.

The biggest message of the impeachment struggle may be that red and blue America are now so deeply polarized that even the compelling evidence of presidential wrongdoing that has been amassed against Trump cannot cross that divide, either to meaningfully move public opinion or to influence the actions of elected officials.

In particular, the decision by virtually all House Republicans to view their role as defending Trump, rather than pursuing a genuine assessment of the underlying facts, underscores how partisan imperatives have almost completely eclipsed any commitment to Congress' independent authority to check and balance the executive branch.

Looking forward to a likely Senate trial, Senate Majority Leader Mitch McConnell , a Kentucky Republican, went on Fox News to announce that he will coordinate every aspect of the procedure with Trump through his White House counsel and GOP Sen. Lindsey Graham of South Carolina has declared: "I'm not trying to pretend to be a fair juror here."

"This is a situation the founders didn't anticipate," says John J. Pitney, a professor of political science at Claremont McKenna College and a former congressional Republican aide. "They thought ambition would counteract ambition and there would be an institutional patriotism in Congress that would counteract the effect of the presidents. But the kind of tribal parties we have now may mean it doesn't work the way they wanted."

Rather than discouraging Trump from again pressing the boundaries of the law, many experts worry, impeachment may embolden him, if his entire party locks arms in both chambers to oppose any sanction for his behavior.

Such a vote, after all the evidence presented from the array of career diplomats and military officials who testified before the House Intelligence Committee, could easily encourage Trump to believe that Republicans will block any meaningful congressional sanction against him, almost regardless of what he does next.

"There is no doubt that the unanimity among Republicans in the Judiciary Committee and the likely unanimity of Republicans in the House means he knows his party is going to stay with him no matter what he does," says Erwin Chemerinsky, the dean of the University of California at Berkeley Law School. "I think the bottom line of all of this, if it plays out the way we are talking about (with no Republicans voting against Trump), it will strengthen the presidency and weaken checks and balances, and that, to me, as a constitutional matter, is very frightening. Because it means there's a president who is immune from congressional oversight and immune from checks and balances."

One party can't defend the norms alone

In all these ways, impeachment could reaffirm one of the most sobering messages of Trump's tumultuous presidency: One party alone cannot defend the norms of democracy and traditional limits on the expansive exercise of presidential power.

Almost always when Trump has pushed against those norms -- from his assertion of "emergency" presidential authority to build his border wall to his efforts to block Robert Mueller's special counsel investigation into Russia's ties to his 2016 campaign to his defiance of congressional demands for witnesses and documents across an array of issues -- enough legislators from his party have supported him to prevent Congress from effectively checking his behavior.

A unified Republican defense of Trump in both chambers over Ukraine would add an exclamation point to this pattern. It would underscore that in this era of unrelenting partisan conflict, legislators -- and for that matter voters -- have grown almost completely unwilling to break from a president of their own party, no matter their behavior.

"If they are not going to impeach and remove him for this, what is there?" Pitney says.

In a world where partisan loyalty so overshadows other concerns, impeachment may not be as strong a sanction as it once appeared. As Chemerinsky notes, by requiring a two-thirds vote in the Senate, the founders always intended the removal of a president to be a very high bar. That was true before our modern era of hyper-partisanship, but what may be changing is how great a deterrent the House can impose by impeaching a president, even if it is unlikely to lead to Senate conviction.

The indivisible opposition to impeachment that Republicans displayed makes it easier for Trump to disparage the process as merely another partisan exercise -- and to find a receptive audience for that argument not only among his own base, but also among some independents who recoil from any kind of elevated partisan conflict.

Polls have shown Americans divided almost exactly in half over whether Trump should be impeached and removed from office. By historic standards, that's substantial support for the unprecedented step of removing a president from office: No more than about a third of Americans ever supported removing Bill Clinton during his impeachment struggle, and a majority backed Richard Nixon's removal only in the final Gallup poll before his resignation.

But Trump's approval rating among Republicans has remained at about 90% or slightly more in many surveys, and roughly the same share of GOP partisans oppose his removal from office. That's encouraged Republican legislators to dig in on their defense of him.

So has the unrelenting opposition to impeachment from the sources of information that conservatives trust most, including Fox News Channel and talk radio. In a recent survey by the nonpartisan Public Religion Research Institute , nearly half of Republicans described Fox as their primary news source and just 2% of them backed Trump's impeachment and removal. Among the other half of Republicans, support reached 10%.

What happened in earlier impeachments

The few other examples of presidential impeachment in American history have also been highly partisan confrontations. But some independent considerations have leavened that mix.

The House voted on a party-line basis to impeach Andrew Johnson in 1868 , but just enough Senate Republicans joined Democrats in opposing his removal to allow him to stay in office.

In 1973 and 1974, the Watergate scandal sharply divided Republicans from Democrats, both in Congress and at the grassroots. But over the investigation, several congressional Republicans such as Sen. Howard Baker sought to excavate the underlying facts and staunchly defended Congress' authority to acquire evidence and witnesses from the President.

During Nixon's final days, about one-third of the Republicans on the House Judiciary Committee voted to impeach him and nearly one-third of Republican voters said they supported his removal, according to Gallup Polls at the time. Nixon resigned in August 1974 after a delegation of senior GOP senators warned him that his support in that chamber was collapsing.

Clinton's impeachment in 1998 unfolded in a more partisan era than Nixon's and reflected the widening distance between the parties. But even then, five House Democrats voted to impeach Clinton and while no Democratic senators backed his removal from office, a much larger number of Democrats in each chamber criticized his underlying behavior in the Monica Lewinsky scandal that triggered the crisis.

On the other side, enough Republicans broke ranks to defeat on the House floor two of the four impeachment articles approved by the House Judiciary Committee.

Tom Daschle of South Dakota, then the Senate Democratic leader, told CNN last week that he did not criticize McConnell for his remarks about coordination and that he had also talked to the White House staff in preparation for Clinton's Senate trial -- though pointedly not to the President himself. But Daschle's loyalty extended only so far: when the Senate refused to remove Clinton, Daschle described the result as no exoneration of the President's behavior. "This was a rebuke. There is no question,″ Daschle said.

Few expect to hear similar criticism from McConnell when this Senate procedure ends.

Standing with the President

The 2019 confrontation has unfolded in a very different manner. While a few congressional Republicans initially raised concerns about Trump's pressure on Ukraine to investigate former Vice President Joe Biden, those voices have been almost entirely silenced as the process has proceeded.

Many analysts have noted that congressional Republicans could have sought to investigate the charges and still determined that Trump's actions did not rise to the level of justifying impeachment or removal. But apart from a few oblique questions from Republican Texas Rep. Will Hurd early in the House Intelligence Committee hearings, GOP legislators never took that first step.

House Republicans on both the Intelligence and Judiciary committees devoted almost all of their energy to discrediting the charges against Trump and disqualifying the witnesses who testified against him, rather than seeking to elicit information from those witnesses about how the pressure campaign against Ukraine folded.

"They were defending him from the beginning," said Pitney. "I think they saw this as a partisan battle, not a constitutional conflict."

"There is no pretense of open-mindedness, no pretense of 'let's hear the facts and then we'll decide,' " Chemerinsky said. "They decided from the outset they are going to stand with the President no matter what the evidence."

Yielding to Trump's defiance

Just as important, Republicans in both congressional chambers have acquiesced to, or even actively supported, Trump's systematic defiance of Democratic demands to produce witnesses and documents relevant to the inquiry. That stonewalling -- and the choice by the legislators in the President's party to support it -- could be among the most lasting implications of this impeachment struggle, experts believe, because it sets a precedent that could allow future presidents to also reject congressional investigatory demands.

Trump's blanket rejection of congressional demands for information during the impeachment inquiry "has set a new standard," Chemerinsky said. "Andrew Johnson and Richard Nixon and Bill Clinton all cooperated with the investigation. His instructing all of his top aides not to go and testify, his refusing to provide any documents, is unprecedented and it really limits the ability to have congressional oversight and checks and balances."

Republicans, in turn, complained that Democrats rushed the impeachment process and failed to provide the administration or congressional Republicans with sufficient opportunities to defend the President. But Trump complicated that argument by refusing Democratic offers to participate in the proceedings while denouncing them as a "witch hunt" or "coup."

John Dean, whose congressional testimony as Nixon's White House counsel helped lead to that President's ouster, said in an interview that the Republican posture toward the impeachment inquiry had been "very different" under Trump than in Watergate.

What the GOP is demonstrating is that "if you don't have one party in control of both chambers, and with a supermajority in the Senate, you can effectively neuter the process," said Dean, now a CNN commentator. "That's what Republicans have been doing."

With Senate Republicans signaling a similarly dismissive posture toward the proceedings, Dean is one of several legal experts who argue that House Speaker Nancy Pelosi should refuse to send over the articles of impeachment until the two parties agree on how to conduct a Senate trial. The declarations of support for Trump by senators such as McConnell and Graham "is contrary to their oath" that they will take before the Senate trial, Dean says. "They take an oath to be impartial in judging the (case). It is not a legislator's oath; it is a juror's oath. How can these people can take the oath?"

No one can express much confidence about how this will play out politically. The lack of Republican voices criticizing Trump definitely creates unease for House Democrats in conservative-leaning districts, though the vast majority of them appear likely to support impeachment anyway.

Conversely, the heightening demand to not only oppose impeachment but also to defend Trump's behavior -- when polls show that a majority of Americans believe he abused his power in Ukraine , whether or not they support his removal -- will stress Senate Republicans facing reelection next year in swing states, such as Cory Gardner in Colorado, Martha McSally in Arizona and Susan Collins in Maine.

(In the new national Quinnipiac University poll released Monday, for instance, fully 95% of voters who disapprove of Trump's job performance said he has abused his power as President, according to detailed results provided by the pollster.)

And in the presidential elections immediately after the past two impeachments -- 1976 following Nixon and 2000 following Clinton -- the president's party lost the White House to an opponent who implicitly presented himself as the antidote to the scandal that ignited the crisis.

The 2020 election could produce the final verdict on whether the modern era of tribal political conflict has defanged impeachment. If Trump loses next year, future presidents may view impeachment as a much greater risk than if he wins. Conversely, if Trump captures a second term, despite the scarlet letter of impeachment, he may conclude that Congress in practice cannot constrain his authority so long as his party remains united behind him in Washington and at the ballot box. From there it's only a short step for Trump to conclude that he was correct last summer when he declared the Constitution's Article II means "I have the right to do whatever I want as President."

UPDATE: This story has been updated to reflect that the House voted to impeach President Donald Trump.