It has been increasingly obvious over the two weeks of the trial that Senate Republicans won't convict President Trump on the articles of impeachment — but that they're also working, in mostly subtle fashion, to keep their powder dry for the next time they want to take out a Democratic president. On Sunday, Sen. Joni Ernst (R-Iowa) acknowledged as much during an interview with Bloomberg News.

"I think this door of impeachable whatever has been opened," Ernst said. "Joe Biden should be very careful what he's asking for because, you know, we can have a situation where if it should ever be President Biden, that immediately, people, right the day after he would be elected would be saying, 'Well, we're going to impeach him.'"

Ernst's comments might confuse close observers of Trump's impeachment. After all, his defenders have made the case that the office of president is beyond — or nearly beyond — accountability to the courts or Congress. That would be true for a Democratic president as well as a Republican, right?

Of course not.

As Republican senators have stepped forward in recent days to explain why they will vote for Trump's acquittal, it has become apparent they are leaving themselves some breathing room to adopt an entirely different rationale in future cases involving a Democratic president. Take Sen. Marco Rubio (R-Fla.). He released a statement on Friday explaining why he will vote for Trump's acquittal — saying the president's ouster "would inflict extraordinary and potentially irreparable damage to our already divided nation."

One would think that standard could apply to the impeachment of any president. Even Richard Nixon had his supporters when he left office. But Rubio left the door open to arriving at a different conclusion in the future: His decision, he noted, didn't depend on Trump's innocence. And Rubio explicitly rejected the Trump team's argument that a president can never be ousted from office on "abuse of power" founds.

Instead, he said, he tried to determine if removing the president from office would be in "the interest of the nation." His answer? "Determining which outcome is in the best interests requires a political judgment." In other words: The right answer is whatever Rubio wants it to be. For a Republican president, at least, the answer is acquittal.

Similarly, Sen. Lamar Alexander (R-Tenn.) didn't even bother to defend Trump's innocence in explaining why he will vote to acquit the president. Instead, he said, the next presidential campaign is already underway. Let the American people work it out.

"The question then is not whether the president did it, but whether the United States Senate or the American people should decide what to do about what he did," Alexander said in a written statement. "I believe that the Constitution provides that the people should make that decision in the presidential election that begins in Iowa on Monday."

Alexander is retiring from office — this is his last bite at impeachment. But his standard also leaves a lot of wiggle room. Does it mean that presidents can do whatever they want, no matter how illegal, so long as it is late in their term? If so, when does the presidential amnesty period begin?

Like Rubio, the most likely answers are whatever Alexander says they are. He is close to Senate Majority Leader Mitch McConnell (R-Ky.) — who famously cited the upcoming 2016 election as a reason not to hold a vote on President Obama's nomination of Merrick Garland to the Supreme Court. McConnell, of course, later admitted he would not apply that precedent to Trump if a SCOTUS opening occurs this year. The Senate's top Republican has no fear of being made to look like a hypocrite — he will do what he wants, regardless. So the answer to all questions about what the GOP will do in a given situation is almost always: "whatever maximizes their power."

Impeaching a Democratic president would certainly maximize Republican power. So it is difficult to believe that Rubio and Alexander accidentally gave open-ended reasoning that could be amended for future votes. Certainly, Ernst's comment confirms that a hypothetical Biden impeachment is still on the table for her party. So did the attempt by Patrick Philbin, one of Trump's impeachment managers, to suggest that abusing power for political gain might not warrant ouster — but that abusing power for monetary gain could be impeachable.

Republicans are going to haunt the next Democratic president, and Democrats had better be ready.

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