Dick Durbin, a Democrat, is the senior US senator from Illinois. The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) To be sworn in as members of the United States Senate, my colleagues and I stand in the well of the Senate, raise our right hands, and pledge to "defend the Constitution of the United States against all enemies, foreign and domestic." Those words are almost a cliché because they are used so often, and, yet, at this moment we are being called on to truly reflect on that responsibility.

This is because of evidence that has come to light in the last several days -- and over the course of the last two and a half years -- that raises serious constitutional questions about President Donald Trump.

Every American understands that it is an attack on our constitutional system for a foreign power to interfere in our democratic elections. Months before the November 2016 election, our nation's top intelligence officials learned of Russia's election interference efforts and informed key Congressional leaders . President Barack Obama sought a bipartisan message condemning Putin's efforts -- avoiding any suggestion of partisanship and presenting a unified front against this Constitutional assault.

Regrettably, the response of the Senate Majority Leader, Mitch McConnell, after hearing this disturbing revelation -- this threat from former communist KGB official Vladimir Putin against our democracy -- was to refuse . Mitch McConnell would not help. He and many in his party said nothing to address this Russian assault -- except for candidate Trump who actually invited Russia to help him defeat Hillary Clinton.

And then for months after the election, Republican Senators largely kept quiet about the accumulated evidence of Russia's attack on our 2016 elections. The silence from Congressional Republicans was noteworthy as President Trump defended Putin's brazen denials of such attacks. The GOP took Putin's word denying being behind any interference over that of our American intelligence professionals.

The Mueller Report later laid out, in exacting detail, how the Russians systematically interfered in our election. The report described "numerous links between the Russian government and the Trump campaign." One would think after such a dramatic finding that any American President would take the threat of foreign election interference seriously and reassure the nation that he really does put America, not a foreign power, first.

But not President Trump. Shortly after the Mueller Report was released, President Trump said that he would still accept a foreign government's offer to share damaging information about a political rival, echoing similar remarks he had made on the campaign trail.

In short, President Trump learned nothing from our national trauma. And the silence of my Republican colleagues seems to indicate the same.

Now we have credible reports that President Trump is at it again, this time trying to pressure leaders of Ukraine to investigate one of his political rivals -- not to advance American interests, not to serve the American people, not to uphold American values, not to help a Western ally -- but to serve his own political interests.

JUST WATCHED 3 House committees subpoena Pompeo Replay More Videos ... MUST WATCH 3 House committees subpoena Pompeo 01:23

On September 12, I offered an amendment in the Senate Appropriations Committee to address suspiciously-withheld US military assistance to Ukraine. As I prepared my amendment, word of my effort spread to the White House, and the night before the vote, the hold on the funds was lifted . Then over the course of the next few days, thanks to investigative reporting and the President's own admissions, we've learned of the President's attempts to use these funds to pressure Ukrainian President Volodymyr Zelensky to cooperate in President Trump's political efforts. We've also learned of a whistleblower complaint reportedly related to the same issue -- a complaint that the Trump administration tried to hide from Congress despite a clear legal requirement that they share it.

Given this history and the gravity of this matter, I called on Tuesday for the initiation of a formal impeachment inquiry by the House of Representatives of President Donald Trump. President Trump's recent actions are beneath the dignity of any President and by any reasonable legal standard merit an impeachment inquiry.

The American people deserve facts. And because the White House refuses to cooperate, answer questions or respond to Congressional subpoenas, a formal impeachment inquiry is the only avenue to pursue this critical investigation.

Get our weekly newsletter Sign up for CNN Opinion's new newsletter. Join us on Twitter and Facebook

This is a trying time for our nation. But to preserve our republic, we must uphold our oath and act to defend our Constitution. In the past, we have had Senators of both political parties stand up to defend the Constitution. And so it is with that in mind that I make a plea to my Republican colleagues: take a stand and make clear that it is unacceptable for a sitting US president to solicit or pressure a foreign country to further any political agenda.

If our oath to defend the Constitution means anything, we cannot ignore this President's actions.