President Donald Trump has accused his Democratic rival Joe Biden of having connections to the “radical left” and has pilloried his relationship with China, his record on criminal justice, his plans for the pandemic and even his son’s business dealings.

But the 74-year-old Trump has so far invested in one line of attack above all: the charge his 77-year-old opponent is too old and mentally weak to be an effective president. The attack has drawn cries of ageism, and there is evidence it may be ineffective. Shrugging off the risks, Trump’s team is pumping millions of dollars into the broadside and vowed this week not to back off.

With Election Day less than four months away, Trump has spent more money on one television ad claiming Biden lacks “the strength, the stamina and the mental fortitude to lead this country” than any other single ad this year.

The 30-second spot and its Spanish equivalent have been running across 12 states, including retirement havens like Florida and Arizona, since mid-June at a cost of $6.5 million, according to data compiled by the media tracking firm Advertising Analytics. The firm noted a shift in recent days toward an unrelated Trump attack accusing Biden of supporting the far-left push to defund police departments, although he said he doesn’t.

Still, the focus on Biden’s age and mental competence continued this week on television sets across the country, backed by a wave of related digital ads asking voters whether Biden “is old and out of it.” A separate television ad produced by a pro-Trump super PAC openly suggests Biden has dementia.

Robert Blancato, who sits on the board of AARP and recently chaired the American Society on Aging, decries what he sees as blatant “ageist” attacks. Speaking for himself, not AARP, he said such attacks don’t belong in politics.

“Not everybody in the world can be a victim of racism, not everybody in the world can be a victim of sexism, but everybody has potential to be a victim of ageism,” he said, predicting ads focusing on age would backfire on the Trump campaign.

Yet while there is no known medical evidence that either candidate is declining, age and mental competence remain a key issue in 2020 for both candidates. Should he defeat Trump this fall, Biden, who turns 78 on Nov. 20, would be the oldest first-term president in U.S. history. Trump, who turned 74 on June 14, holds the current record. Both men are prone to gaffes and rambling when off script.