Through the Constitutional process of impeachment.

Many Republican voters will undoubtedly vote for Hillary Clinton when the alternative is Donald Trump. Few Republicans truly desire such a result. However, if given a choice between Mr. Pence and Ms. Clinton the Republican consensus likely would be for Mr. Pence.

Republican lawmakers could frame that effective choice by agreeing to pursue impeachment proceedings against Mr. Trump following his election. With that step foreshadowed, the Republican electorate would be galvanized to bring about a victory for the Trump-Pence ticket that would result in a Pence administration.

Article II, Section 4 of the Constitution provides that “The President ... shall be removed from Office on Impeachment for … High Crimes and Misdemeanors.” While an initial version of that clause limited the impeachable offenses to “treason and bribery,” a proposal was made to expand impeachable offenses to include “maladministration.” That clause, however, was ultimately rejected in favor of "other high crimes and misdemeanors.” 

While the sufficiency of evidence necessary to sustain articles of impeachment for “high crimes and misdemeanors" remains largely undefined, the term could be applied expansively.

Federalist Paper No. 65, for example, characterizes impeachable offenses as “those offences which proceed from the misconduct of public men, or in other words from the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated political, as they relate chiefly to injuries done immediately to the society itself.”

In 1868, the House impeached President Andrew Johnson for High Crimes and Misdemeanors. One of the principal charges against him was that Johnson removed the Secretary of War, Edwin M. Stanton, a holdover from President Lincoln’s administration, in violation of the Tenure of Office Act that Congress had ostensibly enacted to protect Stanton. Johnson, however, was acquitted in the Senate.

In 1989, the House brought three articles of impeachment against Walter Nixon, a former United States District Court Judge who had been convicted of two counts of making false statements before a federal grand jury. Along with two articles charging him with making false statements before the grand jury, the third article charged Nixon with bringing disrepute on the Federal Judiciary. The Senate, however, convicted Nixon on only the first two articles.

The 1998 articles of impeachment brought by the House against President Clinton were for perjury to a grand jury and obstruction of justice. The House rejected a proposed article of impeachment for abuse of power. President Clinton, of course, was acquitted in the Senate of all charges.

There is slim precedent to establish the conduct necessary to sustain an impeachment. For example, may impeachment proceedings be brought based upon conduct that preceded election to office? Arguably, Donald Trump’s past conduct already has provided sufficient grounds establishing a lack of fitness for office and disrepute to the institution of the presidency.

On the other hand, if he is elected through the popular vote, would that not per se establish his qualification?  But if earlier conduct is insufficient, does anyone truly expect that it would be long before a President Trump provides grounds for concerned lawmakers to bring about an impeachment pursued on broadly defined grounds?

Under ordinary circumstances, no lawmaker would want to stake out a position on a hypothetical and undefined scenario. But if a minimal number of Republican lawmakers declare an intent to vote for the impeachment of Donald Trump if he is elected, the framework of the election would be dramatically altered. Democratic lawmakers, who have pilloried the Republican nominee and incessantly declared him unfit for office, would be hard-pressed not to follow suit and vote to impeach that individual.

An electorate that realizes a vote for the Trump-Pence ticket would not necessarily mean a full Trump presidency, may well turn out to elect that ticket, and give rise to a Pence administration through such proceedings.

Would such a course of action be extreme and extraordinary? Absolutely. But the choice of Hillary Clinton, on the one hand, or an unqualified individual such as Donald Trump, on the other, ascending to the presidency of the United States requires the Republican establishment that midwifed that choice to consider all possible options to thwart such a potentially disastrous result.

Mr. Raskas is an attorney in Baltimore, Maryland.