Editor's Note: (Joe Lockhart was White House press secretary from 1998-2000 in President Bill Clinton's administration. He co-hosts the podcast "Words Matter." The opinions expressed in this commentary are his own. View more opinion at CNN.)

(CNN) With House Speaker Nancy Pelosi's decision to move forward with impeachment, and the White House decision not to participate in the House process, the impeachment of President Donald Trump is virtually sealed.

Joe Lockhart

The only remaining issues moving forward will be how many different articles of impeachment the President will be facing in the Senate -- and what the rules surrounding his trial will be.

It is virtually certain there will be an article on abuse of power, one on bribery for the alleged Ukraine "shakedown" (which Trump denies) and one more on obstruction of Congress for the unprecedented stonewalling of several House committees. What hasn't been decided is whether to add in an additional article on obstruction of justice -- based on evidence presented special counsel Robert Mueller's report.

And while the impeachment vote will be historic, it may lack political drama -- as it's expected to be a party-line vote.

So, the main focus now is on how the Senate will move forward with a trial. There are no rules prescribed in the Constitution; they are set by the Senate itself. The White House has signaled it plans to mount an aggressive defense in the Senate, buoyed by the fact that Republicans are in the majority in the Senate. And Republicans have suggested that Vice President Joe Biden and his son Hunter will be included on their witness list, as they'll try to make the trial about Democratic corruption -- not the alleged abuse of power by Trump.

Given the Republican majority in the Senate, many assume it will run exactly the way the White House wants it to go. But that is a false assumption. Although it takes two thirds of the Senate to remove the President from office, it only takes a simple majority to set the rules for the trial.

And to predict how the Senate will move forward, you have to understand who has leverage and how they will use it.

Republicans sit in a powerful position if they can convince the entire caucus to vote along party lines on the rules. That won't be easy, though, because Republicans in the Senate have competing interests to consider.

There is little doubt that many Republican senators will remain loyal to Trump. But others have a different political concern -- getting re-elected in 2020. Republican Sens. Cory Gardner, Susan Collins and Thom Tillis don't necessarily benefit from a protracted partisan drama on the Senate floor. They all need more than Trump base voters to get elected.

There are also a few Republican Sens. like Rob Portman, Lamar Alexander and Mitt Romney who still have respect for the dignity of the Senate and may balk at the firebrands who want to politically burn down the building.

Finally, there will be the question of where the Majority Leader, Mitch McConnell, comes down. Many still talk about him as an institutionalist, someone who reveres the traditions above the partisan politics of the Senate. Remember back in 1999, when McConnell said of the Bill Clinton impeachment trial -- acknowledging it was unlikely Clinton would be convicted -- "The Senate is on trial here."

But any idea that he's an institutionalist was blown up when he denied former President Barack Obama his constitutional right to appoint a Supreme Court Justice in 2016. McConnell of today will likely only do what he thinks is right for the Republican Party. So, Senate watchers will be looking to see if he's carrying the President's water, or that of the Republicans who are up for re-election. I suspect he cares most about remaining majority leader, and he'll do what's best for his senators.

That alone is leverage for the Democrats. Unlike the House, where the minority party is more like a spectator when it comes to rules, Senate Democrats do have some power here. Going after vulnerable Republicans up for re-election is one.

They also have a little discussed, but potentially powerful tool if they work with their House colleagues. What Democrats want to avoid is a process that gives the President a virtual exoneration on all charges if it's a party-line vote. One thing they could do is ask House Democrats to delay sending the articles of impeachment to the Senate until the White House starts cooperating with Congress or a court compels them to.

What's certain at this point is much of the drama in the Senate trial will begin before Chief Justice John Roberts begins to preside over it -- and that the rules of the trial will dictate the politics of how this all plays out.

There is precedent for the parties coming together and reaching a bipartisan agreement. Still, this is 2019, not 1999. And yet for the Senators who remain from 1999, they know and likely have told their colleagues about the political pain in setting the rules and the importance of what they decide before the trial starts.