Right now it looks very much as if the two major political parties in America are in a race to see which one can destroy itself first.

On the Republican side, Donald J. Trump not only leads but dominates the presidential race. Crude, erratic, unprincipled and unelectable, Mr. Trump, if he were to win the nomination, would do catastrophic damage to the Republican Party. But the Democratic Party, if it were to nominate Hillary Clinton, would be inviting a different kind of disaster.

One clue as to how vulnerable Republicans consider Hillary Clinton to be is the size of the Republican presidential field, 17 — the largest in a century. One explanation for this conga line of candidates is a sense that history is on their side. It’s difficult for the same party to win three consecutive presidential terms. But much of the optimism of Republicans has to do specifically with Mrs. Clinton.

Since the 2008 campaign, it’s been pretty clear that she is, to put it mildly, not a natural political talent. Mrs. Clinton lost a nomination she was heavily favored to win. Her campaign was poorly managed, plagued by indecision, confusion and poisonous infighting. During that run she showed herself to be, in Barack Obama’s withering phrase, “likable enough.”

Things haven’t gotten any better for her since. In public Mrs. Clinton often comes across as inauthentic, charmless and brittle, and she is poor on the stump. When campaigning, one senses that for her it’s an act of will rather than an act of joy. In these respects, she is the antithesis of her husband.

Mrs. Clinton’s supporters point to her experience as a strength. But that can be a double-edged sword; her opponents have some say in which experiences define her. Her first incursion into federal policy was her health care plan, which politically speaking was a disaster. It led to Republicans gaining control of the House in 1994 for the first time in 40 years.

During Mrs. Clinton’s eight years as a senator, she left barely a trace. While she was secretary of state, America’s relationship with our allies worsened, global instability increased and American influence decreased. If Republican presidential candidates are smart, they will use her experience to establish a record of incompetence.

All of this was known before the current presidential campaign cycle began earlier this year. Since then, there has been an intervening event that may well turn Mrs. Clinton from a weak candidate into a crippled one: the stunning revelation in early March that she conducted State Department business on a private email server.

We know from the inspector general of the intelligence community that at least two of her emails contained “top secret” information. According to former intelligence officials, the information on her server was most likely compromised by foreign intelligence services. She not only received but also wrote and sent information now deemed classified. And in trying to cover up her actions, Mrs. Clinton appears to have stretched the truth. The F.B.I. now has Mrs. Clinton’s private server and a thumb drive that had been in her lawyer’s possession. An investigation is underway, and some of the more than 30,000 emails Mrs. Clinton deleted may be recoverable.

And here’s an ominous precedent: Gen. David H. Petraeus, the former C.I.A. director and one of the greatest military commanders of his generation, pleaded guilty to a misdemeanor charge of mishandling classified information, a violation that may prove to be lesser than those Mrs. Clinton may have committed.

Mrs. Clinton has lost control of events as a result of an obsessive need to manipulate them. And Republicans, who already considered her something of an ethical mess, are finding she has exceeded even our expectations.

Mrs. Clinton has already suffered significant political damage. Her once formidable leads against Republicans and her primary Democratic challenger, Bernie Sanders, have narrowed. In a recent Quinnipiac University Swing State Poll, Mrs. Clinton’s favorability ratings in Ohio, Florida and Pennsylvania were very negative, while her honest and trustworthy numbers were in the low 30s. Another Quinnipiac University poll asked voters what’s the first word that comes to mind when you think of Hillary Clinton. The first three were “liar,” “dishonest” and “untrustworthy.”

If nominated, Mrs. Clinton will be the weakest Democratic nominee since Michael Dukakis in 1988. But many Republicans, presented with this golden opportunity, are enamored of a man who, if he’s the nominee, will be their weakest since Barry Goldwater in 1964. The Republican Party has a much deeper and more impressive field than the Democrats do, so they still have plenty of alternatives. But as summer gives way to fall, one of those alternatives had better emerge. The Democratic nominee will be beatable, but not if Donald Trump, unreason personified, is the unappealing face of Mr. Lincoln’s party.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center, served in the last three Republican administrations and is a contributing opinion writer.