How to prevent Donald Trump from ascending to the presidency:

Hillary Clinton should reach out to all of her 232 electors to cast their vote for Paul Ryan as U.S. president in the electoral college, which is held on December 19th, 2016 in the House of Representatives. Hillary should also reach out to all of Trump's 306 electors to vote for Ryan as well.

There is no downside for Hillary Clinton to pursue this strategy, since the worst case scenario is that Trump wins anyway. The upside is that there is some probability of electing Ryan (or possibly Mike Pence), thereby denying Trump the presidency.

Here's an illustration:

If at least 38 of Trump's 306 electors are willing to defect to Ryan and all 232 of Hillary's electors vote for Ryan, then Ryan would receive 270 electoral votes, which is a majority, and Ryan would be elected president by the electoral college, instead of Donald Trump.

If at least 38 of Trump's 306 electors vote for Ryan, but too few Hillary electors support Ryan to give Ryan a total of 270 electoral votes, then no candidate would have a majority of electoral votes, causing the election to go to the House of Representatives, which would choose from among the 3 candidates who received the most electoral votes as outlined in the 12th Amendment to the Constitution. Depending on the preferences of Republican members and how they are distributed across different states, the House would choose either Trump or Ryan. If the House were deadlocked, then the Vice President (presumably Mike Pence) would become acting president until the House resolved the deadlock.

If 36 (or fewer) of Trump's 306 electors are willing to defect from Trump, then Trump gets 270 electoral votes and becomes president.

If this strategy by Hillary Clinton gave the presidency to Paul Ryan, it would deny the presidency to Donald Trump, who is singularly unfit, extreme, threatens to take rights away, and permanently damage our country's institutions. In his Presidential campaign, Donald Trump has promised to force all Muslims to register with a U.S. government database, to torture people accused of terrorism, to kill innocent family members of people accused of terrorism, and to end birthright U.S. citizenship. Any one of these monstrous proposals would cast a cloud of shame and intolerance on the United States for the rest of history. Everyone who loves this country's values of freedom and inclusion has a duty to prevent these proposals from being enacted -- even through unconventional means, like the strategy described above -- and to prevent anyone who believes in them from coming to power.

Why Paul Ryan, and not a Democrat (like Hillary Clinton) or a moderate Republican (like Mitt Romney)? Because it is impossible to convince 38 Republican electors to switch from Trump to a moderate Republican, much less a Democrat, over whom virtually all 306 Republican electors prefer Trump. However, it may be possible to convince 38 Republican electors to switch their vote to a conservative, like Paul Ryan, whom many Republican electors might prefer over Trump. Moreover, in the second scenario described above where the election goes to the House of Representatives, Republican House members might choose Ryan over Trump but would never choose Romney (or Clinton) over Trump. The House can choose only from top 3 vote getters in the Electoral College, two of whom will be Trump and Hillary, so there is only one remaining slot to be filled. Hillary can win neither the Electoral College vote nor the House vote. Unless that third slot is filled by someone who can win either the Electoral College vote or the House vote, we are stuck with Trump. In summary, given the preferences of the electors chosen in the November 8th election and the preferences of House Republicans, Paul Ryan is our only feasible alternative to Trump.