Historians of the presidential horse race have plenty of mud to wade through, from Watergate to railroad bribery, and plenty of sex scandals in between.

But this year's election offers something new: Donald Trump appears to be the first nominee of a major political party in modern memory to be the subject of ongoing litigation, according to presidential experts.

"I don't know of any other litigation involving a major candidate for election," Columbia University scholar Henry F. Graff told NBC in an email, when asked about the fraud lawsuits over Trump University.

Graff, who has presented his book "The Presidents: A Reference History" to presidents Reagan, Clinton and George W. Bush, is one of over a dozen political scientists and scholars in the field interviewed for this article. Most said the legal and ethical issues that Trump and Hillary Clinton bring to the 2016 election are largely unprecedented as well.

"There is a cloud of legitimacy surrounding them which opens the door for more opposition," said Julian E. Zelizer, a political historian at Princeton.

Trump faces three lawsuits over his now-defunct Trump University; the suits claim he defrauded the people who signed up to learn his real estate secrets, only to be promised better results with more expensive packages. He denies the charge, and some former students have praised the school, though his criticism of the presiding judge in one case, an Indiana native whose parents are from Mexico, fanned accusations Trump discriminates against Hispanics.

The first case is scheduled to be heard shortly after voters cast their ballots for president in November.

"It would probably take a little bit of the gleam off of his election if he were seen as fleecing the same people who he says on the campaign trail he's championing," American University professor Leonard Steinhorn said.

If Trump won the election, losing the Trump University lawsuits wouldn't kick off a constitutional crisis — they are civil cases, not criminal, and wouldn't make him ineligible to serve as president.

And even though the lawsuits are apparently unprecedented, the experts said they may hardly affect his campaign at all.

Traditional politicians might become damaged goods after being dragged through the mud of a lawsuit, but Donald Trump has proven to be the most successful outsider in Americans politics in at least a generation, and he's emerged largely unscathed from many lawsuits already, from discriminatory housing practices to sexual misconduct.

Asked for comment, Trump campaign spokeswoman Hope Hicks only said Clinton is the first candidate to be interviewed by the FBI in a criminal complaint. (A Clinton representative declined to comment. Special prosecutors did interview Bill Clinton and George W. Bush during their re-election bids.)

FBI Director James Comey didn't recommend bringing charges against Clinton, but he said her team was "extremely careless in their handling of very sensitive, highly classified information," and revealed findings that contradicted some of Clinton's statements about the extent of her private email use.

"The emails have already hurt Clinton," said Joseph Cummins, whose book "Anything for a Vote" chronicles each presidential election through 2004. "If the attorney general came out and scolded (Trump) I don't think it would hurt him."

TEFLON DONALD?

To longtime Democratic strategist Bob Shrum, now a professor at USC, Trump's legal history is an important proxy for judging his character. Where Clinton can be measured by her years of public service, Trump "has no political history."

"You're taking somebody who's in the rough and tumble of business (so) his business practices are the only thing we have to judge for his candidacy," Shrum said.

Trump's career as a property developer practically demands the occasional visit to court, and his business left a very long legal trail. He's been sued at least 150 times in federal court alone since the early '80s, according to an Associated Press review of court records.

A USA Today investigation published in June found Trump has been involved in 3,500 lawsuits overall, which the report said was "unprecedented for a presidential nominee" and uncommonly frequent for a developer.

Lawsuits are usually deterrents for people seeking office, noted Hoover Institution research fellow Bill Whalen, who called Trump the most litigious candidate for president ever.

So are scandals. Trump's dating life in New York in the 1980s and '90s was tabloid newspaper fodder, sometimes front page news; on the other hand, a White House affair got Bill Clinton impeached in 1998.

"We want our president to build his or her presidency on a strong ethical foundation, on the bonds of trust of who they are with the American people," said Steinhorn, the American University professor.

And yet, none of the presidential experts said they felt Trump University would definitely sink his campaign. Yes, it could be an "anchor," as Cummins, the author, put it, but "it's the type of thing that Trump always survives."

Trump's ready-made defense is that "he was conducting a business," Steinhorn said.

And Trump's supporters have probably already made up their minds about the nominee, and won't be swayed if they learn more about the Trump University allegations, according to Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania.

"Evidence has indicated new information won't change the minds of decided voters. They will rationalize the information," she said.

In fact, Trump hasn't faced much scrutiny for Trump University. Most of it came soon after he said Judge Gonzalo Curiel had a conflict of interest because Trump's proposed border wall would keep Mexicans from illegally entering the U.S.

Since then, Trump turned the focus from his own legal case to Clinton's email woes, insisting it was criminal so often it's become the dominant political story of the month.

The lack of legal resolution makes for a deeply negative summer to come, with the allegations an important tool for both campaigns, yet potentially very damaging as well.

"Both sides are worried that they won't have high turnout, that this will turn voters off," said Zelizer, the Princeton professor.

Their choice in this election will be between a Republican nominee with civil litigation hanging over his head and a presumptive Democratic nominee reeling from a criminal investigation that won't go away.

'BURN THIS LETTER'

With a candidate dogged by a scandal in one corner and a man with checkered personal life in the other, the election of 1884 might offer the best analogy to 2016's.

It pitted Republican Sen. James Blaine, of Maine, against Democrat New York Gov. Grover Cleveland, who nearly lost despite Blaine being a part of the biggest scandal of the 19th century, according to convention historian Stan M. Haynes.

Blaine had waited two election cycles to become the Republican nominee, ever since a set of letters scuttled his presidential aspirations in 1876 by backing up the claim he was paid $64,000 for bad railroad bonds. It was deeply, damagingly unethical, according Haynes, but not illegal.

He surged again in 1884, and won the nomination, but new letters came out, this time to railroads explaining exactly what to say to exonerate him.

"He wrote 'Burn this letter' at the bottom," Haynes said. "The chant against him was, 'James, James, James G. Blaine, continental liar from the state of Maine.'"

Cleveland might have soared to the White House on Blaine's scandal, but he came crashing down to earth when he was accused of fathering a child out of wedlock some years before (he admitted paying child support). The Republican chant against him was "Ma, Ma, where's my Pa?"

In the end, Cleveland won by a hair. If just 600 people in New York had voted for Blaine instead, the state and the election would have swung for the Republicans.

There is a fairly recent example of a candidate being sued, according to Cummins, the author. But Ross Perot wasn't in a major party, and he was actually sued for dropping out of the 1992 race, by a campaign worker who "felt that he had an obligation to people that worked for him." The suit was dropped.

Perot rejoined the race and won nearly 20 million votes in the general election, despite being a political independent, making him the most successful outsider in modern politics, at least until 2016.

But Perot also split the conservative vote with George H.W. Bush, helping Bill Clinton take the White House. The Trump campaign will try to stop his wife from winning the presidency in another year of the political outsider, hoping voters believe their case against Clinton and don't heed the legal cases against Trump.

Kate Guarino and The Associated Press contributed to this article.