JEWEL SAMAD/AFP/Getty Images Presumptive GOP presidential nominee Donald Trump speaks in New York on May 3, 2016, following the Indiana primary.

If Donald Trump weren't actually the presumptive Republican nominee, the 2016 race would probably be shaping up to be pretty competitive, say those who predict election results for a living.

As political scientist Julia Azari wrote earlier this week:

Election forecasts based on what political scientists like to call the 'fundamentals' — the state of the economy, how long the incumbent party has been in office, and how popular the incumbent is — are usually pretty accurate. Second, polarization has emerged as a powerful force in American politics. With an electorate that’s pretty set in its party preferences, it’s not too hard to figure out what the vote might look like.

Working in the Democrats' favor, President Barack Obama's job approval rating, which languished in the red for most of his second term, now looks as healthy as it did during his 2012 re-election, and views of the economy have modestly rebounded.

On the other hand, voters may be more than ready for a change after sticking with one party for two terms, which would (theoretically) give the GOP an advantage.

Attempts to model elections give these factors different weights. Economist David Rothschild, the founder of the forecasting site PredictWise, says a "generic" Republican candidate should have a 47 percent chance -- or just under even odds -- of winning the presidency. Another model, created by a team of political scientists, gives the GOP a more bullish 60 percent chance of winning the election.

But 2016 is different, and Trump is anything but generic.

Predictions based on fundamentals carry the assumption that both candidates are on a relatively even footing -- what political scientists John Sides and Lynn Vavreck liken to a game of tug-of-war in which "both sides are pulling equally hard." Yet while both the real estate mogul and Democratic hopeful Hillary Clinton are historically unpopular, Trump is overwhelmingly so, with polls putting his unfavorability rating at an average 60 percent.

“In terms of any domestic personality that we have measured, we’ve never seen an individual with a higher negative,” Democratic pollster Peter Hart said in March, describing Trump's image as "exceptionally rancid."

If a mainstream Republican candidate were the presumptive nominee, the GOP would likely be in a strong position for a lot of wins. Ed Goeas and Brian Nienaber, The Tarrance Group

Trump is also straining party unity in a way that Clinton doesn't seem to be. Partisan concerns may eventually win over Republican voters and political dignitaries, but Trump's nascent general election campaign has already inspired a wave of high-profile GOP holdouts to condemn him.

House Speaker Paul Ryan said Thursday that he wasn't ready endorse Trump. The two most recent Republican presidents, George W. Bush and George H.W. Bush, have said they'll skip the Republican National Convention, as have previous GOP nominees Mitt Romney and John McCain.

By contrast, Clinton has yet to sew up her primary or win over Sen. Bernie Sanders (I-Vt.), but she's already won the backing of much of the Democratic establishment.

"This was never the Democrats' race to win," Sides said in an email Tuesday. "I think as of right now, Trump's negative favorables make him less likely to win than you'd expect given the fundamentals."

Other measures of the race, including horserace polls and prediction markets, back up that sentiment. PredictWise's Rothschild gives Trump just a 30 percent chance of winning the election. HuffPost Pollster, which includes all publicly available polling, shows Trump starting off 7 points behind Clinton nationally, and trailing her in a number of crucial states.

"Although we remain convinced that Hillary Clinton is very vulnerable and would probably lose to most other Republicans, Donald Trump's historic unpopularity with wide swaths of the electorate - women, millennials, independents and Latinos - make him the initial November underdog," forecasters at the Cook Political Report said Thursday, explaining why their map shows swing states like Florida, Colorado and Virginia leaning blue.

A recent George Washington University Battleground poll made it especially clear how the presidential race might be different if Trump weren't involved.The survey split voters into two groups: Half were asked to pick which of the two parties they trusted most to handle a range of issues, while the other half were asked specifically about Clinton and Trump.

Voters gave Republicans a substantial lead over Democrats on jobs, taxes and the economy, and a smaller edge on foreign policy. Adding Trump's name to the mix immediately ceded that advantage.

"The Republican Party has a strongly favorable political environment for winning the White House. If a mainstream Republican candidate were the presumptive nominee, the GOP would likely be in a strong position for a lot of wins, top to bottom, in November," pollsters Ed Goeas and Brian Nienaber, from the GOP polling and research firm The Tarrance Group, wrote when the survey was released.

"However," they added, "the party is facing the challenge of the possibility of an unconventional nominee who is likely to squander most of this favorable environment due to his temperament and rhetoric."

Editor's note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims -- 1.6 billion members of an entire religion -- from entering the U.S.