The largest cost is likely to show up in the wake of a Clinton win. The narrower her margin of victory, the easier it will be for Mr Trump to spark outrage over a stolen election. It would also give him more of a grip over Republicans in Congress. Legislators respond to feedback from their districts. By Jeffersonian design, the House is where the link between elected and elector is most electric. If Mr Trump's base is inflamed, Republican legislators will take their cue. Most of their voters already believe Mrs Clinton is dishonest and corrupt. It is no great leap to endorsing Mr Trump's claim that the Clintons are a "criminal enterprise".

It is hard to talk yourself down from such incendiary language. It would be easier were the target anyone other than Mrs Clinton. Before re-entering the White House, she would be the most investigated political figure in US history. Not even her husband has been the subject of this many probes and subpoenas . If she wins next week, Mrs Clinton will face many more. With the help of WikiLeaks, Republicans believe they have enough ammunition to open up fresh investigations. For ambitious legislators, holding her feet to the coals is a sure-fire route to folk-hero status.

Unwitting ally

Their unwitting ally is Mrs Clinton herself. It is remarkable that she has not yet promised to sever family links to the Clinton Foundation if elected. At the moment the plan is for Chelsea Clinton to take over its day-to-day running. It is not enough for Caesar's wife to be clean. She must be above suspicion. The Clinton Foundation fails that test. It has taken billions of dollars from governments, companies and wealthy individuals - some dubious. That the cheques were made out for good causes is no help. To Mrs Clinton's foes the foundation provides a cornucopia of stories and scandals. Governments such as Qatar and Morocco channelled money through Mr Clinton, rather than the UN, say, or the Gates Foundation for a reason. In their view it provided access to, and possible influence over, Mr Clinton. He and Mrs Clinton earned big speaking fees from some of the same entities.

In a highly charged environment, Mrs Clinton would take office as the first president since Gallup started polling to have a trust deficit with the American public. Mrs Clinton is both the victim and co-author of her own torment. Every time she took a decision with any bearing on a former donor to the Clinton Foundation, there would be a perceived conflict of interest. That would apply even more to entities that have paid private speaking fees to either Clinton. Potential conflicts would not just feed the Republican mill. If a Clinton-appointed regulator dismissed some charge or other against Goldman Sachs, would the Democratic left believe it? If she approved a hefty arms sale to a Gulf state, could it be taken at face value?

It is not too late for Mrs Clinton to promise to mothball the foundation. The longer she leaves it the harder it will become. It is not just her husband's pride that is at stake, or her daughter's future. This is about her ability to govern America. edward.luce@ft.com

Bill Clinton on the hustings for his wife. Suspicions of influence-peddling through the family foundation have dogged the campaign. Justin Sullivan

Financial Times