Julian Zelizer, a CNN political analyst, is a professor of history and public affairs at Princeton University and author of the forthcoming book, "Burning Down the House: Newt Gingrich, the Fall of a Speaker, and the Rise of the New Republican Party." Follow him on Twitter: @julianzelizer . The views expressed in this commentary are his own. View more opinion at CNN.

(CNN) The White House transcript of President Donald Trump's phone conversation with Ukrainian President Volodymyr Zelensky should cause Republicans to tremble. It reveals that the President did push Zelensky to investigate Democratic candidate Joe Biden. Before asking Zelensky to "do us a favor" by helping with this investigation, he reminded him that the US has been "very, very good" to Ukraine.

This kind of stunning evidence of the President abusing his power is the reason that House Speaker Nancy Pelosi finally took the major step of throwing her full support behind the impeachment inquiry. The six committees currently investigating President Trump are now operating under one umbrella with the official mandate of determining whether to vote for articles of impeachment. Should they end up taking this vote, Trump would join a small list that includes only three other presidents who have faced impeachment since the establishment of the country (Andrew Johnson, Richard Nixon, and William J. Clinton).

The President has struck back in predictable fashion. "A total Witch Hunt!" and "PRESIDENTIAL HARASSMENT!" were some of the first words to come in over the Twitter transom . A number of Republicans have already joined him. At the heart of the counterattack is the claim that impeachment is a radical action. The entire process, Republicans will likely continue to say, is being driven by a Democratic party motivated by its left-wing base. Trump will try to defeat Speaker Pelosi and the Democrats as President Clinton prevailed over Speaker Newt Gingrich and the Republicans in 1998

If Democrats want to retain the support of their moderate wing who have joined this cause and desire to stand any chance of winning over Republican support in the House or Senate, they will need to respond to these charges of harassment. Fortunately for Speaker Pelosi, in the case of Trump, supporters of impeachment have a series of cogent arguments to make to convince GOP members of Congress.

While much of the discussion has centered around the Democrats, the spotlight should be on the GOP. They are the party being tested. And there are four reasons why there is a clear conservative case for the impeachment of President Trump.

1. Restraining the power of presidents

Restraining presidential power should be a top priority for conservatives. Few recent presidents have exercised their power with the same level of reckless abandon as the current commander in chief. If Republicans mean what they say about championing limited government, then there really is no justification for the party's decisions since the time of Richard Nixon to continue granting power to the executive branch when their party was in power. Simply put, further empowering the President means growing the role of government. The fact that the President has so much authority, from the ability to send troops into combat without a Congressional declaration of war to using executive measures to achieve policy goals like building a wall, does not square with their agenda.

Unlikely as it may seem, supporting impeachment is an unprecedented opportunity to reverse course on their hypocrisy and re-embody their own ideals. Trump has exposed, more than almost any of his recent predecessors, just how much a president can do if they are willing to ignore basic norms and if they are not scared about the potential response from Congress or the courts. Supporting the impeachment process, and possibly a decision to impeach and convict, could allow Republicans to go on record showing that they still do believe in small government.

2. Protecting national security

Protecting national security has also been a central goal of the conservative movement. During the 1980s, for instance, President Ronald Reagan railed against Democrats for being weak on defense as he pushed for higher defense spending and insisted on taking a tough stand with the Soviets. George W. Bush pushed for a proactive militaristic response to terrorist networks as well as states that he claimed threatened American interests. Yet President Trump has not done much to uphold this tradition. His coziness with Vladmir Putin has frustrated intelligence and military officials who understand that Russia has been conducting ongoing efforts to interfere in our electoral system.

The President has handled relations with North Korea in topsy-turvy fashion, doing little to produce an actual nuclear weapons agreement or halt North Korea's weapons production. His efforts to persuade Ukraine , possibly withholding military aid as an incentive to investigate Trump's 2020 rival Joe Biden, have inspired similar fears. All of this prompted seven moderate Democrats who are veterans of the military to publish their Washington Post piece endorsing the impeachment inquiry.

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3. Standing up for the rule of law

Republicans have also loved to imagine that they are the party of law and order. Ever since Richard Nixon's time in office, they have tried to assure voters that they are the party that can be best trusted to follow the rule of law and to give law enforcement institutions all the support they need. Under the Trump administration, they have so far been sitting passively as a President ran roughshod over all the institutions they have claimed to champion. Throughout his presidency, Trump has peddled mendacity and offered disinformation contradicting solid intelligence data that has been collected on matters like Russia. He has repeatedly acted with disregard for the law, he's been accused of flagrantly obstructing justice while under investigation by Mueller and refused to cooperate fully with congressional oversight.

The current scandal was triggered by the fact that the administration did not share the whistleblower complaint with Congress, as is required by law. The accumulation of evidence that the President does not abide by the rule of law is overwhelming. Republicans can only show Americans they still care about this crucial aspect of our democracy if they cooperate with this investigation.

4. Mike Pence

In addition to these three core conservative elements behind the impeachment investigation, Democrats looking to forge bipartisan accord can reiterate that if President Trump were impeached and convicted, he would be replaced by an extraordinarily conservative Republican -- Vice President Mike Pence -- who is arguably even more rightward than the President. So if this really is a left-wing effort to mount a coup, it is poorly conceived. Success would not change the balance of partisan power nor would it in any way moderate the agenda of the White House.

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Based on the voluminous evidence that we already have of Trump's wrongdoing, there is a compelling argument that impeaching him could be the only way to demonstrate the GOP's genuine commitment to conservative principles. In some ways, the most damaging step the Republicans could take is to continue stifling the investigation and blindly defending the President even as most can see before their very eyes the way that he is running roughshod over everything they have claimed to stand for.