Why are so many Democrats hellbent on shunning bipartisanship and ramming trillions in spending through Congress on party-line votes? Why are they pushing to rid of the filibuster so they can pass the most radical elements of their agenda over Republican objections — even though it would strip them of the ability to block GOP legislation when they return to the minority?

For the answer, just look at the Supreme Court's 7-to-2 decision to uphold Obamacare.

The Affordable Care Act, which Senate Democrats passed in 2010 on a party-line vote, has faced three Supreme Court challenges and multiple efforts to repeal it in Congress. Yet more than a decade later — despite periods of unified Republican government and the appointment of three conservative Supreme Court justices — the law is still standing.

The lesson for Democrats is clear: Government is a one-way ratchet. Once a new entitlement program is created, it never gets dismantled. So, if you believe in big government, compromise is foolish. You should go for broke whenever you have power — because while doing so may cost you power temporarily, the gains you make will be permanent.

Many Democrats knew they were risking political suicide by voting for Obamacare. In the 2010 midterm elections that followed its passage, Republicans gained six seats in the Senate and 63 seats in the House — the largest shift since 1948. A 45 percent plurality of voters said that they cast their vote as a message of opposition to Obamacare. And the losses kept coming. During President Barack Obama's term, the number of Democratic senators dropped from 60 to 46, while the number of Democrats in the House fell from 257 to 188. But Democrats were willing to sustain those losses because they knew the fruits of their sacrifice would never be undone — and they were proved right.

This is why so many Democrats have no interest in reaching across the aisle today. They understand they don't have a mandate for socialism — voters elected a 50-50 Senate and the narrowest House majority in decades. But they also know that if they create new programs and entitlements such as free higher education, government-funded child care, universal pre-K and paid family leave, they will never be repealed. If Democrats lower the age at which Americans can receive Medicare, Republicans will never raise it. If they create a "public option" for health care, it will be irreversible. So, they want to enact as much as they can, as quickly as they can, while they still hold the reins of power. And if doing so costs them power, so be it.

Make no mistake, Democrats don't want to lose power. That is why they are so frustrated with Sens. Joe Manchin III (D-W.Va.) and Kyrsten Sinema (D-Ariz.), who are blocking their efforts to get rid of the legislative filibuster. The filibuster not only stands in the way of their most radical agenda items, it also prevents them from building a firewall against the inevitable conservative backlash. With the filibuster in place, Democrats can't create two safe Democratic Senate seats by making D.C. a state, or pass a partisan election law that will make it harder for Republicans to win back power.

But even if Republicans manage to win back the House, the Senate and eventually the White House, Democrats know it will be only a temporary setback — because the GOP will not enact irreversible accomplishments of their own. If Republicans cut taxes, Democrats will raise them when they return to power. If Republicans cut budgets and repeal regulations, Democrats will restore them. If Republicans pass protections for unborn children, Democrats will simply repeal them. Other than judicial appointments, there is little Republicans can do in power that Democrats cannot undo when they regain it. That's because of a difference in philosophy: Democrats seek to expand government, while Republicans seek to limit its size and scope. So, the worst that happens under GOP rule is a temporary pause in the expansion of government. Then Democrats return and pick up where they left off — and the inexorable march toward socialism starts up again.

This is why Democrats are willing to risk their majorities by doing things that go far beyond the mandate Americans have given them at the polls. They know if voters punish them by electing Republicans, what they do in power will be temporary and reversible, while what Democrats do is lasting and permanent.

Marc Thiessen writes a twice-weekly column for The Post on foreign and domestic policy. He is a fellow at the American Enterprise Institute, and the former chief speechwriter for President George W. Bush.