President Biden signaled last week that Isaac Newton’s third law of motion — for every action, there is an equal and opposite reaction — may finally apply to American politics.

Do not underestimate the significance of Biden’s statement at a CNN town hall Thursday that he has reached the end of his rope on a Senate filibuster that has been used to increasingly destructive effect in the years since Barack Obama came to office.

“We’re going to have to move to the point where we fundamentally alter the filibuster,” said Biden, a longtime skeptic of changes in the filibuster.

Biden asserted his willingness to push for bypassing the filibuster to allow Senate Democrats to enact a voting rights bill — and went on to say he might favor lifting it on behalf of other measures as well.

Until now, the asymmetry of our nation’s political contest seemed to defy Newton’s proposition.

Republicans in the Senate have relentlessly used their power to scuttle Democratic proposals or to make passage excruciatingly difficult — witness the painful and protracted negotiations over Biden’s Build Back Better program.

GOP aggressiveness has not been met by a comparably ferocious Democratic response. Except for a small tweak in filibuster rules in 2013 initiated by then-Senate Majority Leader Harry M. Reid (D-Nev.) to ease confirmation for executive-branch appointees and lower-court judges, Democrats largely stood by as GOP obstruction ran rampant.

Meanwhile, Sen. Mitch McConnell (R-Ky.) had no compunction during Donald Trump’s presidency when he led the majority to set aside the filibuster for the cause he cares about most: the confirmation of right-wing Supreme Court justices.

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What finally moved Biden — and also longtime skeptics of changing the rules, among them Sens. Angus King (I-Maine) and Jon Tester (D-Mont.) — is how recent Republican actions have fundamentally altered the terms of the debate. The need for filibuster reform was brought home by the solid phalanx of Republican opposition last week even to debating a voting rights bill. That followed a recent GOP threat to use the filibuster to block efforts to lift the debt ceiling, normally a routine action.

Opponents of filibuster reform, notably Sen. Joe Manchin III (D-W.Va.), have spoken of wanting to protect the Senate’s best habits of open debate, a degree of bipartisanship and honest give-and-take.

But abuse of the filibuster has itself become the central threat to a functional Senate and the civil behavior extolled by those most devoted to its traditions. This means we have reached what might be called a Burkean Moment. Edmund Burke, the founding philosopher of modern conservatism, insisted that preserving what we value most can require, even demand, reform. Those who revere the Senate should pay heed.

The way Senate Majority Leader Charles E. Schumer (D-N.Y.) has handled the voting rights issue helped drive us to what White House press secretary Jen Psaki on Friday called this “inflection point.” Manchin has said repeatedly that any bill about elections would be better if it could pass with bipartisan support. So Schumer encouraged him to put forward proposals of his own and then shop them around to Republicans in search of 10 votes to break a filibuster.

The Freedom to Vote Act is a compromise measure written to Manchin’s specifications. It pared back many of the provisions that Republicans had objected to in the For the People Act passed by the House in March.

Despite Manchin’s best efforts at persuasion, not a single Republican was willing to support it.

In the meantime, more than a dozen Republican-led states, on party-line votes, have enacted a slew of voter suppression and election subversion measures that the Manchin-backed compromise bill is designed to remedy. Democrats, including Manchin, confront a moment of truth: They can fight for democracy or they can keep the Senate’s filibuster rule as it is. Republicans have made very clear that they cannot do both.

When the Freedom to Vote Act was blocked last week, Schumer was careful to describe the stakes in terms that Manchin and other skeptics of Senate rules revisions would pay attention to. “The Senate needs to be restored to its rightful status as the world’s greatest deliberative body,” Schumer said. “Members of this body now face a choice — they can follow in the footsteps of our patriotic predecessors in this chamber. Or they can sit by as the fabric of our democracy unravels before our very eyes.”

It was Theodore Roosevelt — the Republican Roosevelt — who described resistance to reform as representing “not true conservatism, but an incitement to the wildest radicalism.” That is where we are with voting rights and the filibuster. Joe Biden has drawn the obvious conclusion. Senate Democrats need to join him.