As each day of the Intelligence Committee’s inquiry passes, it seems more likely that the House will deliver articles of impeachment to the Senate. At that point, the Constitution requires two-thirds of senators to vote to remove President Trump from office. That supermajority vote can happen only after a trial, over which Chief Justice John G. Roberts Jr. will preside.

The last Senate removal trial took place 20 years ago, after the Republican House charged President Bill Clinton with perjury and obstruction of justice. Our research on that trial suggests that the Senate is unlikely to convict President Trump today.

Here’s how we did our research

House rules require majority support to send articles of impeachment to the Senate. Democrats control the House; recently, they voted 232 to 196 along party lines to open an impeachment inquiry. That suggests that the House is likely to impeach.

But in the Senate, it’s a different story. The Constitution’s framers imposed the two-thirds requirement — a very high bar ­— for extraordinary actions like overriding a presidential veto, ratifying a treaty, adopting an amendment to the Constitution — and removing a president.

In 1999, exactly half of the Senate found Clinton guilty of obstruction of justice, while the other half voted to acquit. In a second vote, 55 senators found Clinton guilty of perjury. Neither vote came close to the two-thirds mandate.

In our research, we found that legislators were driven by ideology — and not legal arguments — when voting on Clinton’s removal. We estimated the ideology of senators serving during the Clinton removal trial and compared their ideological positions to how they voted on removal of the president. We also examined senators’ verbal statements on removal based on the legal and constitutional arguments they made in speeches.

Because President Richard M. Nixon resigned before the House could vote to impeach him, we have no comparable data from that era. This makes the Clinton case even more instructive today.

Senators’ ideologies matter more than their legal arguments

The political ideology of senators mattered far more than legal and constitutional arguments in predicting the outcome of the Clinton trial. The most conservative senators voted to remove the Democratic president; liberal members did not. The kinds of legal arguments senators made did not correlate with their votes. Legislator ideology was the reason senators voted to acquit or to convict the president.

To remove Clinton, pro-removal Republicans would have needed “guilty” votes from liberal Democrats. They didn’t get them.

It will take conservative GOP senators to convict Trump

If senators’ ideological commitments still predominantly shape their votes, we can assume that all or most Democratic senators will vote to remove Trump. That leaves the president’s fate up to Republican senators. Democrats will presumably first try to persuade GOP moderates like Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska) that Trump’s behavior in office has included high crimes and misdemeanors. But that won’t get Democrats close to the supermajority required to unseat him. If every Democratic senator and these two moderate Republicans vote to convict President Trump, only 49 senators would support removal.

To reach the 67-vote threshold, Democrats would also need to persuade conservative Republicans. We looked at data on senators’ ideologies, and found that the most likely candidates for those senators at the pivotal 67th-vote position in the Senate include a group of conservatives such as Richard Burr (R-N.C.), Lindsey O. Graham (R-S.C.), Johnny Isakson (R-Ga.), Jerry Moran (R-Kan.), Pat Roberts (R-Kan.), Rick Scott (R-Fla.), and John Thune (R-S.D.). These Republicans, along with 13 other more moderate Republicans, would have to vote “guilty” to actually remove Trump.

Here, ideology is captured by DW-NOMINATE scores, which fall along a -1 to +1 scale. Positive values indicate more conservative and negative values more liberal senators. Moderates are near zero on the scale. This set of pivotal conservatives near the 67th-vote pivot point in the Senate have scores from 0.4 to 0.5. By contrast, the most moderate Republican in the Senate, Susan Collins, has a score of just 0.1.

These pivotal Republicans are solidly conservative senators who often vote with the president on substantive issues. But if they don’t want to remove Trump from office, it won’t happen.

So what are they saying? So far, they show little interest in convicting the president. Thune has supported Trump robustly — although in October, he did tell reporters that the picture coming out of the now infamous July 25 call phone call between Trump and Ukrainian President Volodymyr Zelensky was “not a good one.”

Graham has become one of the president’s most vociferous supporters. He once said that proof of a quid pro quo on aid to Ukraine would be “very disturbing.” However, this pivotal conservative is refusing to read transcripts of the testimony and has called the impeachment inquiry in the House “a bunch of BS”

When Burr was chairman of the Senate Select Committee on Intelligence, Trump praised him for ending the Russia inquiry. But the president and his congressional allies were annoyed when, in May, Burr’s committee issued a subpoena to Donald Trump Jr. Unlike Trump and Graham, Burr now says that the whistleblower who touched off the impeachment inquiry should be kept anonymous.

Further, Burr is retiring, and so is Roberts. Since these pivotal senators wouldn’t have to explain their votes to constituents, they might feel free to vote according to their personal views about the charges and the evidence Democrats muster.

Watch their ideologies, not their arguments

If — or more likely, when — the Senate holds a trial, senators will make a lot of legal claims to justify their votes. Our research suggests that those explanations are secondary to the ideological leanings of senators. Observers should pay close attention to these pivotal conservatives as impeachment and removal move forward in Congress, and less attention to the legal claims. The only way for Trump to be removed is for these conservatives to vote him out.

The chance that Trump will be impeached is high, and the chance that he’ll be found guilty in the Senate is not zero. But if ideology decides the outcome as it did for Clinton, the chance that Trump will be removed from office is quite low.

Anthony Bertelli (@tonybertelli) is the Sherwin-Whitmore chair of Liberal Arts and professor of public policy and political science at Pennsylvania State University and professor of political science at Bocconi University.

Christian Grose (@christiangrose) is associate professor of political science and public policy at the University of Southern California and is the academic director of the USC Schwarzenegger Institute.

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