With a House impeachment vote a foregone conclusion, the battle to remove President Trump from office has moved to the Senate. Minority Leader Chuck Schumer grabbed control of the debate Monday with demands for what he called "fairness" in the president's trial.

Schumer wants the Senate to allow testimony from four witnesses the House did not interview: former national security adviser John Bolton, acting White House chief of staff Mick Mulvaney, key Mulvaney aide Robert Blair, and Office of Management and Budget official Michael Duffey. House Democratic impeachers wanted the men to testify, but after the White House, claiming privilege, refused, House leaders chose not to try to force them to appear. Going to court to compel their testimony, Democrats said, would take too much time.

Now, Schumer wants the witnesses simply to forget about privilege questions and testify in the Senate trial.

"How, on such a weighty matter, could we avoid hearing this, could we go forward without hearing it?" Schumer asked at a news conference Monday. "I haven't seen a single good argument about why these witnesses shouldn't testify — unless the president has something to hide and his supporters want that information hidden."

Republicans will respond that the Senate is not the place for fact-finding — that is, for senators to become investigators and do what the House declined to do. Some will also note that the House chose not to seek the appointment of an outside investigator, a special counsel, to establish what happened in the Trump-Ukraine matter, and the Senate is ill-equipped to play that role. Many will also argue that the facts of the case do not align with the Democratic accusation of bribery, and more testimony will not change that. Others will argue that they don't believe what the president did rises to the level of an impeachable offense.

In other words, they will come up with a number of good arguments, perhaps not good arguments to Schumer, but good arguments to Republicans, not to have any more witnesses.

Schumer is not trying to convince all 53 Senate Republicans to support his proposal. He just needs four. There are 47 Democrats in the Senate. If Schumer can persuade four GOP senators to join Democrats, they'll have a majority of 51 and can force the calling of new witnesses. Of course, Schumer is counting on Democrats voting as a bloc against the president, which is probably a good bet.

If Schumer gets what he wants, it seems hard to believe that will be the end of it. The request for more witnesses appears designed to lead not to closure but to reopening the case against Trump. In this way, if Democrats can introduce new testimony in the trial, they can say the new testimony has raised new questions that will require new investigation. And a new investigation will require more new witnesses, which will surely lead to more new questions, which ...

Call it the Brett Kavanaugh model of impeachment. During the Supreme Court justice's confirmation process, a hearing had already been held, and Kavanaugh appeared on the way to joining the court. Then, up popped a new allegation, the Christine Blasey Ford story, and Democrats demanded the case be reopened, witnesses be interviewed, evidence be gathered, and time be taken for more investigation. Republicans acceded to those demands, and the Kavanaugh confirmation careened off course for a while before GOP lawmakers finally got it back on track.

It is not beyond imagination that a Senate trial, were it controlled by Schumer and his Democratic colleagues, might take a similarly unforeseen course. Beyond new witnesses, there are plenty of ways that could happen.

For example: On Monday, lawyers for the House Judiciary Committee told a federal court it is essential that grand jury materials from special counsel Robert Mueller's Trump-Russia investigation be given to House investigators. Why? Because it might help the impeachment effort. "If the House approves articles of impeachment," the lawyers argued, "relevant grand-jury material that the committee obtains in this litigation could be used during the subsequent Senate proceedings."

If the Judiciary Committee receives the information, there is little doubt its leaders will work with Senate Democrats to create the impression that there is new evidence so compelling that it absolutely must be included in the impeachment trial. And if Republicans disagree, what are they trying to hide?

Indeed, Schumer and his colleagues have prepared the way to characterize any move to limit the trial's scope as an effort to hide wrongdoing from the American people. "To engage in a trial without the facts coming out is to engage in a cover-up," he said Monday.

The bottom line is: Republicans should not believe for one minute that the campaign to remove the president will rely only on the case Democrats have built in the House. Schumer and other party leaders will scramble for new information to throw at the president, and at Republicans, until it is over. The GOP, and the White House, need to be ready.