If the Democratic Party keeps surging to the left and nominates a 2020 candidate who can be credibly labeled “socialist,” “radical” or just far out of mainstream, it will not only reelect Donald Trump, but unleash a cascade of nightmares on itself and the nation.

Unpopular as he is, President Trump likely would win narrowly, but whatever the margin, he will feel vindicated and unleashed. To do what? For sure, to continue treating unwanted immigrants with systematic cruelty, claiming to help the working middle class while further enriching the wealthy, offending foreign allies while coddling dictators, dividing the nation and demeaning political discourse.

Other dire scenarios await if the party keeps heading left, responding to its liberal base and the pressure of TV debates. One possible outcome is that Democrats will lose what slim chance they have of retaking the Senate. Then Trump will be able to nominate and confirm probably at least one more and possibly two more conservative Supreme Court justices, plus lower court justices and executive branch officials who’ll carry out his will.

It gets worse. Democrats recaptured the House in 2018 because 40 moderates took seats Republicans won in 2016. If the party is deemed far-left in 2020, Democrats could lose control of the House and Trump could once again operate without any serious congressional oversight.

No one seems to realize the danger more than Speaker Nancy Pelosi, who has been resisting liberal pressure to begin impeachment proceedings that would inflame Trump’s base.

And she has been trying to tame the most left-wing elements of her caucus, the four-member so-called “Squad” led by first-term socialist Rep. Alexandria Ocasio-Cortez, who has been lionized by the media and is being used by Trump to brand the entire Democratic Party as radical.

Pelosi clearly is motivated by her past experience as speaker from 2007 to 2011, when she acted as a “San Francisco liberal,” epitomized by her forcing Obamacare through the House on a party-line vote. As a result, Democrats lost 63 seats in 2010 and she lost her speakership.

Besides Trump unleashed, chances of winning the Senate lost, a more conservative Supreme Court and loss of the House majority, a fifth potential nightmare scenario for Democrats is another decade of GOP domination at the state level.

In the post-Obamacare Democratic wipeout election of 2010, Republicans gained 680 state legislative seats, control of 19 legislative bodies and won a net six governorships. In the following year’s reapportionment, they used their authority to gerrymander U.S. House and state legislative districts to give themselves commanding advantages through the current decade.

If the same thing happens in 2020, not only would the gerrymandering continue in 2021 — making it harder yet for Democrats to win back the House -- but GOP-dominated legislatures could further restrict abortion, block gun control, impose new voting restrictions, and cut back on Medicaid and other social programs.

Various commentators, including Thomas Edsall of the New York Times, GOP political guru Karl Rove and Democratic centrist William Galston in the Wall Street Journal have described how perilously far Democrats are heading out of the mainstream of American opinion.

Among the items that could well make it onto the 2020 general election platform are “Medicare for All” (including cancellation of employer-provided private insurance and reimbursement levels so low as to drive hospitals out of business); a Green New Deal proposing a close-down of fossil fuel use in just 10 years; immigration policy that effectively opens U.S. borders to the world by decriminalizing unauthorized entry and abolishing the Immigration and Customs Enforcement agency; endorsement of no-limits abortion; allowing prisoners to vote; and enlargement of the U.S. Supreme Court.

Other items being bandied about by the 2020 Democratic presidential field include free college for all and elimination of college debt regardless of income, providing free health care to undocumented immigrants, various threats to “take on” wealthy Americans and large corporations through break-ups and higher taxes, a guaranteed income for people whether they work or not, a huge low-cost housing construction program and subsidies for low-income renters, a return to a 70% top income tax rate and a “wealth tax” on people with net worth over $50 million, reparations to all African Americans, and free child care and preschool for all, regardless of income.

Then there is the left’s ever-growing demand for Trump’s impeachment, favored by only 22% of voters (including only 36% of Democrats). Opening impeachment proceedings might strengthen Democrats’ investigative authority, but actual impeachment would be a vain exercise.

Trump is certain to be acquitted in the Republican-controlled Senate and would claim not only personal vindication but proof that Democrats have never accepted the voters’ choice in 2016.

Risking all this is utterly unnecessary. Democrats don’t need to explore the leftist frontier to beat Trump and win at other levels. They can simply advocate policies the public overwhelmingly favors.

These include keeping the current U.S. health care system (including private insurance) and adding a Medicare “public option.” Also, controlling prescription drug prices, which Trump promised to do but didn’t.

The public also approves giving legal status or a citizenship path to undocumented immigrants and treating asylum-seekers humanely (but not abolishing ICE or decriminalizing illegal entry).

Government’s rebuilding of infrastructure — which Trump promised to do but never did -- is highly popular. Building Trump’s wall is not. The public supports increasing taxes on the wealthy (perhaps even Warren’s wealth tax, but not a 70% top rate).

According to Gallup, by 60% to 33%, Americans oppose repeal of Roe v. Wade. Only 25% believe abortion should be legal in all cases, 13% in most cases, 39% in only a few, and just 21% say it should be illegal in all cases.

By 60% or more, the public believes that climate change is real, caused by humans and needs reductions in fossil fuel use over time and investment in alternatives. But voters are split on AOC’s aggressive Green New Deal.Gradually raising the minimum wage is popular, as are other programs for the poor and middle class, such as increasing the child tax credit and the Earned Income Tax Credit. A guaranteed income whether one works or not is less popular. Free college for everyone is not popular, but loan forgiveness based on income is. Sixty percent of voters favor stricter gun laws and an assault weapons ban and more than 90% back universal background checks.

Bottom line: The public already backs progressive policies — reasonable, affordable ones, but not far-left ideas. This suggests that a candidate in the moderate range could win -- Joe Biden, Amy Klobochar, Michael Bennet, John Delaney or Steve Bullock (maybe Pete Buttigieg, though he favors court-packing), but Bernie Sanders, Elizabeth Warren, Cory Booker and Kamala Harris would have a tougher road.

Democrats should remember what’s happened when they nominated perceived far-outers in the past: In 1972, George McGovern, labeled (even by some Democrats) as the candidate of “acid, amnesty and abortion,” lost 49 states to Richard Nixon.

In 1984, Walter Mondale, a liberal who supported raising taxes and freezing U.S. nuclear weapons at a level giving the Soviet Union an advantage in Europe, again carried only one state against Ronald Reagan. And in 1988, ”proud” ACLU member Michael Dukakis, perceived as soft on crime, lost all but 10 states to George H.W. Bush.

Meantime, Democrats have won by nominating perceived moderates or moderate liberals Jimmy Carter, Bill Clinton and Barack Obama.

Can Democratic primary voters heed the lessons of the past (and public opinion) and beat Trump, or will they let their ideological enthusiasms run away with them? The consequences of that choice are epic.