In a court of law, a party on the receiving end of civil or criminal charges can move to dismiss those charges if he can prove that the charges do not state a case for which relief can be granted.  This argument can be made either on the ground that the law does not recognize the claimed cause of action or criminal charge or on the grounds that the facts stated in the opening papers fail to meet the requirements for such a cause of action or criminal charge.  The president's opening papers show that his attorneys will argue that the articles of impeachment fail under both of these metrics.

For some time, however, Republicans have been concerned that the Senate will not allow the equivalent of a motion to dismiss.  Instead, they've fretted that the Senate will give credibility to the fatally flawed articles of impeachment by allowing a full evidentiary trial.  Even assuming that Trump were to prevail in the impeachment proceedings, his opponents would still argue that the claims were valid enough to merit a full hearing.

It appears, though, that Senate majority leader Mitch McConnell is leaving the door open to allowing Trump's attorneys to move to dismiss on the grounds that the House Democrats have not made either a factual or a constitutional case against President Trump:

Senate Majority Leader Mitch McConnell is preparing a resolution that would leave room for President Trump's lawyers to move immediately to dismiss the impeachment charges if they so choose, according to Republican Sen. Josh Hawley.

[snip]

"I am familiar with the resolution as it stood a day or two ago," Hawley, the junior senator from Missouri, told me in a phone interview on Saturday. "My understanding is that the resolution will give the president's team the option to either move to judgment or to move to dismiss at a meaningful time..."

• Hawley added that in the most recent draft of the organizing resolution he saw there was an option for the president's counsel to make a motion in multiple places, including at the beginning of the proceedings.
• A Republican leadership aide responded: "The White House has the right to make motions under the regular order, including a motion to dismiss, right after the resolution is adopted because a motion to dismiss is a motion permitted by the impeachment rules."

Some Senate members have already stated in advance that they will not vote to dismiss without hearing the case on the merits.  Some believe that the Republicans' small majority makes a motion to dismiss pointless.  Others believe that Trump should have the opportunity to present substantive evidence to vindicate himself.

And others, despite being Republican, dislike Trump and hope a parade of hostile witnesses, no matter how pointless their evidence, will hurt Trump.  This would be the Kavanaugh strategy of throwing huge amounts of mud in the hope that it makes Trump look dirty, no matter how clean he is.

The American people should hope for a motion to dismiss.  If it's a good motion, it will allow the government to drop this farce and move on to real business.  Of course, given the House's Democrat control, maybe it's a mercy that its Trump obsessions have kept it from legislating.  It's probably best for America if this particular Congress doesn't produce many laws.