At stake for the nation starting Tuesday in the U.S. Senate are matters of importance greater than whether President Donald J. Trump survives in office. Although Trump is the one facing impeachment, it is the continued viability of the Constitution’s checks and balances that is at risk.

The House sent two articles of impeachment to the Senate last week. The first charges Trump with abuse of power for holding up $400 million in military aid to Ukraine to try to force it to investigate the son of Trump’s political rival, Democratic presidential candidate Joe Biden.

The second, more constitutionally consequential article, charges Trump with obstruction of Congress for blocking witnesses and documents from the House investigations. The constitutional issue is whether a president can ignore congressional oversight with impunity.

The facts are not in dispute. What the Senate must decide is whether those transgressions warrant Trump’s removal.

Before they begin these weighty deliberations, senators will vote Tuesday on an organizing resolution to establish the rules for the trial. How they decide these procedural matters holds great import for the relevance of the Constitution.

At issue is whether to limit the trial to only the evidence submitted from the House investigations, or whether to invite additional witnesses and evidence to be admitted.

New evidence emerges daily on the tangled Ukraine affair. It is of paramount importance that both the Senate and the American people learn every available detail.

Late last week, four Republican senators — Maine’s Susan Collins, Alaska’s Lisa Murkowski, Utah’s Mitt Romney and Tennessee’s Lamar Alexander — voiced support for a Senate vote to call witnesses. That vote would take place after the House managers prosecuting the impeachment presented their case and the President Trump’s lawyers argued his defense.

Senate Majority Leader Mitch McConnell (R-Kentucky), a reliable defender and enabler for Trump, wants no witness testimony and no new evidence submitted beyond what the House — which, remember, had its own probe inhibited by Trump’s obstruction — was able to include in its two articles of impeachment.

Tuesday’s procedural decisions could prove pivotal. McConnell, if he holds together enough of his Republican majority, could get rules approved blocking any path to later witnesses.

Our editorial board has strongly endorsed the push for new witnesses and evidence that would provide greater insight into the events.

Former National Security Advisor John Bolton, acting White House chief of staff Mick Mulvaney, and Secretary of State Mike Pompeo could provide direct, first-person knowledge. Bolton said he is willing to testify.

Democrats are united in pressing for witnesses. If at least four Republican senators join the bloc of 45 Democrats and two independents, the Senate could have a true trial with the introduction of evidence and testimony.

Trump has said he would invoke executive privilege to prevent anyone in his administration from testifying. He can invoke executive privilege all he wants, but the power to subpoena witnesses and compel them to testify in an impeachment of the president rests exclusively with the Senate.

Trump’s position poses a constitutional threat, however. If the Senate caves to Trump’s claim of absolute executive privilege, Congress will be surrendering its constitutional authority, as an equal branch of government, to hold the executive branch accountable.

Supreme Court Chief Justice John Roberts may play a pivotal role presiding over the trial. Roberts will “rule on all questions of evidence,” according to the Senate’s standing rules. That includes the validity of Trump’s executive privilege claim.

Roberts is not the final word, however. Whatever decisions he makes can be overruled by a majority vote of the Senate.

In the end, whether Bolton testifies or not, the outcome of the Senate trial appears to be a foregone conclusion. Removing Trump from office requires 67 Senate votes; an unlikely outcome given the Republican majority.

That may be so. But the country needs – and deserves – a trial in the Senate that is perceived as thorough and credible.

The Day editorial board meets with political, business and community leaders to formulate editorial viewpoints. It is composed of President and Publisher Timothy Dwyer, Executive Editor Izaskun E. Larraneta, Owen Poole, copy editor, and Lisa McGinley, retired deputy managing editor. The board operates independently from The Day newsroom.