Democratic presidential candidate Joe Biden has said that if he gets elected, "we're going to cure cancer."

Similarly, President Donald Trump announced at his re-election campaign kickoff rally this week that "w e will come up with the cures to many, many problems, to many, many diseases — including cancer."

But cancer experts say a potential cure for cancer is decades away, and that pretending it can be done sooner isn't helpful to the public or to science.

President Trump previously proposed slashing cancer research funding.

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WASHINGTON — As a crowd of 20,000 looked on, laughing, Donald Trump Jr. on Tuesday mocked his father's emerging rival Joe Biden for the ambitious pledge he made recently to "cure cancer" if elected president.

"I'm going to cure cancer," Trump Jr. said contemptuously, throwing his arms above his head. "Wow! Why the hell didn't you do that over the last 50 years, Joe?"

Once President Trump took the stage at his campaign kickoff rally in Orlando, he made his own pledge: He, too, would cure cancer once and for all.

"We will come up with the cures to many, many problems," he said. "Many, many diseases — including cancer."

But cancer, like health care, is complicated. Discoveries cannot be predicted, research is rarely linear, and scientists often require false starts before they learn how to overcome particular roadblocks in research. The pledges from Biden and Trump, however well-intentioned, experts say, threaten to give the public false hope without accelerating science.

Supporters attend President Donald Trump's re-election campaign kick-off rally in Orlando, Florida, on June 18, 2019. Reuters

Curing cancer is complicated, since it's a class of more than 200 diseases

In a 2020 election dominated by health care, Democratic primary candidates have struggled to differentiate themselves. Already, Biden has leaned on his son's 2015 death from brain cancer and his advocacy surrounding the Obama administration's 2016 "cancer moonshot" as a means of connecting with voters. Trump's pursuit of certain childhood cancer initiatives — and his echoing of Biden's lofty goal — likely indicate that cancer research and care will remain a prominent issue throughout the campaign.

Still, some scientists bristle at putting a timeline on finding a "cure" for anything.

"It's a great aspirational goal, but cancer is an extremely complicated disease," said Jill O'Donnell-Tormey, the CEO of the Cancer Research Institute, a nonprofit focused on cancer immunotherapies. "It's not just one disease. It's 200-plus diseases."

Researchers at drug companies and universities across the country are still making fundamental discoveries about how cancer spreads — underscoring just how much remains unknown about the basic biology of the disease. While some futuristic treatments that involve turning immune cells into cancer-killers exist, they work only for select patients.

The result: For some cancers, no treatments exist that can extend a person's life more than a few years. It is unlikely that reality would change by the end of Trump's presidency, or a challenger's first term, in 2024.

The truth is, even the most seasoned scientists are reluctant to predict exactly when cancer might be cured. When Eric Lander, who co-chaired the President's Council of Advisors on Science and Technology during the Obama administration, offered up a timetable three years ago, he suggested major advancements would be measured in decades, not years. "It's not going to be all done in 10 years,'' he said, "but if we get it done in 40 years, I'm not going to be embarrassed."

What is useful, advocates say, is simply funding research, including the basic inquiries in cells and animals that lay the foundation for new medicines, and translational studies that show whether a drug candidate actually extends patients' lives. They also point to getting more people enrolled in clinical trials and advancing diagnostic and prevention strategies as ways to improve cancer care.

Still, researchers say attention from the country's leaders is always welcome.

Biden's "focus during his final year as vice president working on the cancer moonshot has had a monumental effect," said Jon Retzlaff, the chief policy officer at the American Association for Cancer Research. "But curing the more than 200 diseases that we call cancer is not a realistic goal."

Dr. Edward Sickles (right) and Larisa Gurilnik look at films of breast x-rays at the UCSF Comprehensive Cancer Center on August 18, 2005 in San Francisco, California. Getty Images

Biden and Trump aren't the first politicians to make this lofty promise

Some advocates want presidential candidates to outline plans beyond additional research funding and promoting scientific collaboration. Fran Visco, the president of the National Breast Cancer Coalition, said the status quo has too often resulted in drugs that barely extend a patient's life — and that still aren't affordable.

"Just giving more money to people who are going to do what they're already doing is not going to solve the problem," Visco said.

By promising to cure cancer, Biden and Trump are echoing a half-century of such vows, dating back to Richard Nixon's "War on Cancer" in 1971, a year before his reelection campaign. Biden, in declining to run for president in 2016, lamented that he would not become "the president that ended cancer," just over a year following the death of his son Beau.

On funding and scientific engagement, Biden's track record differs dramatically from Trump's. Biden oversaw the Obama administration's "cancer moonshot" project — an initiative that provided billions of new dollars for cancer research.

Upon leaving office, he founded the Biden Cancer Initiative, a nonprofit aimed at accelerating cancer research and pushing private companies to improve the experience of cancer patients nationwide.

Trump has pushed for less cancer-research funding, not more

More recently, researchers celebrated when Trump, in his 2019 State of the Union address, proposed spending another $500 million on pediatric cancer research. Experts said the money would help them pursue specific questions, including how to extend the promise of immunotherapies to pediatric cancers.

But overall, Trump has proposed reducing funding for the National Cancer Institute by $900 million, part of his efforts to cut government-funded scientific research. Congress has broadly ignored these proposals, instead boosting funding for the National Institutes of Health, of which NCI is part.

"For the Trump administration to propose a 12 percent cut for the National Institutes of Health … and then to say let's go try to win the war on cancer, it's inconsistent," said David Arons, the CEO of the National Brain Tumor Society.

Once, Trump falsely claimed that noise from wind turbines causes cancer.

Along with Nixon, former Vice President Al Gore and former President George W. Bush also raised curing cancer during their political careers. As for why candidates make such lofty vows, it may boil down to how widespread and deeply the impact of cancer has been felt.

"It's a major problem, it's a major health issue, and it's a problem that probably touches almost everyone on the planet," O'Donnell-Tormey said. "And I think it does require a commitment from our government, that this is a priority. People want to hear that this is a priority."