You can already hear the rumbling in the distance — a train of noisy liberal Democrats barreling straight for the White House. They should arrive just in time for President Barack Obama’s second inauguration.

The president already has his hands full dealing with angry and unrealistic Republicans. Now he’s getting reacquainted with their counterparts on the left — a less ideologically inflexible bunch but not necessarily any more susceptible to reason.

Recognizing the enormous stakes in the 2012 election, liberals took the advice of Dr. Evil and “zipped it” during the entire campaign. They refrained from any criticism of the president, lest it help Republican challenger Mitt Romney.

For a party famous for its lack of discipline, that was impressive. So was the Obama campaign field organization. Humorist Will Rogers once said, “I’m not a member of any organized political party. I’m a Democrat.” Lately, given the disarray on the Republican side, Rogers might have swallowed hard and seen fit to declare himself a Republican.

If Democrats are better organized than in the past, they still have their foibles. Recall the crowd at the Democratic National Convention in Charlotte, North Carolina, loudly booing Los Angeles Mayor Antonio Villaraigosa as he left the stage.

Villaraigosa, chairman of the convention, had just claimed that two-thirds of the delegates had approved by voice vote the reinstatement in the party platform of a provision supporting Jerusalem as the capital of Israel. It was more like two delegates and maybe a third — about the same tiny level of support that welcomed Obama’s insistence that “God” be put back in the platform, too.

Before the campaign, liberals were hardly hesitant to express their disappointment with the president. Recall the liberal unrest of 2009 when Obama, bowing to congressional pressure, failed to close the prison at Guantanamo Bay, Cuba, and neglected to support a public option in the Affordable Care Act.

Liberals crying “kill the bill” came dangerously close to derailing landmark health-care reform for which they had been fighting since the Progressive “Bull Moose” Party Convention of 1912. Obama rightly complained in response that too many of his supporters were letting “the perfect be the enemy of the good.”

Now we’re about to see such imperfection under assault again. While Obama won strong Democratic backing for the so- called fiscal-cliff deal in both the Senate and the House, a chorus of liberal critics rose up to condemn his compromises.

They were particularly incensed that he agreed to raise the threshold on income subject to a higher tax rate from his oft- stated preference of $250,000 per family to $450,000 per family. Some news stories reported that Obama broke a campaign promise by abandoning the $250,000 level.

A few liberals even complained that Obama violated his principles by compromising. They must not have listened to him all year. One of his most important — and most frequently stated — principles is that compromise is essential to governing.

Having said that “not everybody gets a hundred percent of what they want” from negotiations, Obama surely would have doomed these and future negotiations by clinging to every jot and tittle of his opening offer.

Perhaps Republicans, too, have now been forced to take the plunge into pragmatism. One achievement of the fiscal-cliff deal was that it violated the “Hastert Rule,” named for former House Speaker Dennis Hastert, that required “a majority of the majority” Republican caucus to proceed on legislation. Instead, Republicans split on the vote and the bill passed with Democratic support.

Just as Republicans must learn to live with tax increases, Democrats must learn to live with — and vote for — changes in entitlements. They should keep in mind that reforms such as a chained consumer price index, which alters the inflation calculation applied to Social Security, and means testing the benefits of wealthy retirees, do not threaten the social safety net.

Neither Franklin Roosevelt on Social Security nor Lyndon Johnson on Medicare was wedded to any of the particulars of those programs — only the principle of guaranteed support from the government.

The road ahead is paved with compromises that many Democrats won’t like. The president will stick to his refusal to negotiate with Republicans who want to hold an increase in the debt ceiling hostage to spending cuts. But he will have to negotiate over the sequester — the $1.2 trillion in cuts to defense and domestic programs scheduled to take effect in two months.

Decoupling the debt ceiling from the sequester will be daunting, if not impossible. Even if Obama succeeds, he will have to agree to cuts to entitlements or discretionary programs, a course many liberals oppose. They haven’t forgotten how Obama almost betrayed their interests in the failed “Grand Bargain” talks in July of last year.

If liberals are disappointed in Obama’s fiscal-cliff deal, imagine how they will feel in late February when he starts making tough choices on spending cuts. Liberals need to think harder about what their own long-term deficit reduction plan would be. Raising more revenue is necessary. It’s not sufficient.

(Jonathan Alter is a Bloomberg View columnist and the author of The Promise: President Obama, Year One. The opinions expressed are his own.)

Photo credit: AP/Charles Dharapak