What if Donald Trump loses the election and blames "cheating" by a rigged system? He has been laying the groundwork for a sore loser campaign since August, and this has escalated as his odds of victory waned.
This is a most dangerous path, as it could incite his supporters to violence, and feeds mistrust of the political process. Hillary Clinton's ability to govern may be limited by persistent whispers about her election.
In particular, after Election Day Republican officeholders will be faced with competing impulses: to acknowledge the legitimacy of Clinton's victory, and the broader set of electoral institutions responsible for their own victories, or side with Trump's fervent supporters against the establishment.
If Trump alleges that specific voting fraud (and not broad structural disadvantages for his campaign like this or this) that led to his defeat in a critical number of states, there is a process for congressional Republicans to challenge the elections. I suggest that it would be in the best interests of Congress and the nation to force a vote on the issue.
Congressional challenges in history
As long as there have been elections, there have been allegations of voter fraud. For presidential elections, Congress has the power to prevent invalid electoral votes. The Constitution specifies that Congress shall open and count the electoral votes received from the states (and Washington, DC).
But what if Congress challenges a state's electoral votes and cannot decide? Congress faced this situation in 1877, in the wake of the disputed presidential election of Hayes versus Tilden. Most historical accounts of this election focus on the decision of an ad hoc blue-ribbon commission to award the votes of three contested Southern states to Hayes. Less noted is the next step: Southern Democrats filibustered the vote-counting process in the US House, creating a constitutional crisis resolved (allegedly) by the Compromise of 1877.
To prevent similar crises in the future, Congress enacted a law in 1887 to allow challenges to the electoral votes from a specific state if one member of the House and Senate each agrees to the challenge. Once a challenge is made, each chamber debates the challenge for two hours (hence no filibustering) and then votes, with the challenge prevailing only if it wins a majority in both chambers. (Information from a US House historian and this passage from my first book.)
In 2000, several House members attempted to challenge the electoral votes from Florida, but no senator would co-sponsor their challenges, so the president of the Senate — Al Gore — had to rule them out of order. In 2004, there was a formal challenge to the electoral votes from Ohio, followed by a debate and vote in the House and Senate:
Almost all the votes for the challenge came from the Congressional Black Caucus, with the rest of the Democrats affirming the outcome of Ohio's vote (if not the methods). In this way, the protest was made while the result was accepted.
Dealing with a Trump challenge
In the event that Clinton wins and Trump alleges that the election was "stolen" from him by some kind of cheating, it is in the interest of Clinton, the Democrats, and the electoral process to bring those claims to a vote. In the alt-right media cycle, an anecdote becomes a scheme, a scheme is a conspiracy, and through unchallenged repetition, the conspiracy becomes a coup. Far better to subject the claims to open debate and to put Republicans on the record affirming or rejecting these claims.
For the Republicans, such a vote would provide a critical early test of their individual and collective futures. Will they follow Trump to the end, or stand up to their party's fringe and return to conventional politics?