In his mind, Harry S. Truman’s famous desk sign motto, “The BUCK STOPS here!” defined a president’s responsibility. “No one else can do the deciding for him,” he explained.

“Responsibility” also implies “moral, legal or mental accountability,” according to our Webster’s dictionary.

Without an outbreak of that kind of responsibility in just three offices on Capitol Hill, the United States may soon default on the national debt, a prospect that leaders in Congress have merely put off until December.

President Biden will not be responsible for that in either sense of the word, although some may be scheming to make it seem so.

Those actually responsible would be Mitch McConnell, the Senate Republican leader, and his two Democratic enablers, Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona.

Should these senators be unable to get a long-term deal done in a few months — and McConnell has already said Democrats can expect no help out of him — our economy would no longer be the world’s most reliable, our dollar no more the international standard of stability, much to the delight of Russia and China. There would be vast microeconomic effects as well, including rising mortgage and auto loan payments and delayed or diminished Social Security checks and military salaries.

Moody’s Analytics estimated that default would cost some 6 million jobs and erase up to $15 trillion — yes, that’s with a T — in household wealth.

As Biden put it, “A meteor is headed to crash into our economy.”

Both political parties have contributed to the debt, which must constantly be refinanced as treasury notes come due. Until now, both have taken their turns at accepting responsibility for raising or suspending a ceiling that ought to have been simply eliminated long ago.

What’s different this time, and much more dangerous, is that McConnell is stopping the essential legislation. He wants to force the Democrats to enact it through the filibuster-proof budget reconciliation process, which Biden and others say is too slow and too complicated.

Democrats never filibustered the debt ceiling bill.

McConnell’s game is to make it appear to voters that the Democrats and Biden are solely responsible for a rising debt even though no Democratic president contributed as much to it as Donald Trump did.

McConnell evidently sees his life’s purpose as the destruction of Democratic presidencies. He couldn’t be so potent at this moment, however, if it weren’t for Manchin and Sinema. His leverage is their unwillingness to abandon or even modify the filibuster rule, which now allows a minority of 41 senators to stop a debate before it starts. That’s a gross perversion of the original purpose of allowing unlimited debate.

The filibuster will also empower McConnell to kill the “Freedom to Vote” legislation that even Manchin co-sponsors.

The Senate’s 50-50 partisan split, which requires Vice President Kamala Harris to break any ties, has given Manchin and Sinema more influence, even without the filibuster factor.

Their opposition to the $3.5 trillion social spending plan supported by the president and most Democrats provoked House progressives into blocking passage of the bipartisan infrastructure bill, putting Biden’s entire domestic agenda in peril.

Manchin’s arbitrary insistence on spending less than half the $3.5 trillion is not his only extortionate price. He’s also demanding inclusion of the anti-abortion Hyde Amendment, which is a non-starter for House Democrats and an outrageous affront to the majority of American voters.

The fate of the infrastructure bill, which Manchin claimed as his own and boasted of what it would bring to West Virginia, is now effectively dependent on the legislation he is opposing. He, not Biden, is responsible for that. There was no other way for the House Democrats to call his hand.

Because life is not fair, presidents can expect to be held accountable not just for what they do but also for what they cannot control. In that, they are vulnerable to the expectations they create for themselves.

Biden expected the nation to have controlled the coronavirus by now.

Some of the decline in popularity is his fault.

The public’s expectations of Biden were inconsistent with the scenes of people falling from airplanes at the Kabul airport or people at the Texas border.

His decision to not pass the buck on Afghanistan to a fifth president was started by the bad deal Trump made without consulting the Afghan government. Most Americans understand that and agree it was time to end our longest war.

But public opinion is holding him to account for the failure of the diplomatic, military and intelligence agencies to foresee how swiftly the Afghan government would collapse.

He should have anticipated for himself how the mass expulsion of migrants would fracture his relations with many Democrats.

Biden needs to own those failures and move on.

They pale, however, beside the disaster for which McConnell, Manchin and Sinema must want to be responsible.