President Obama concluded his big Syria speech on Tuesday night with arguments explicitly addressed to his "friends on the left."

I ask you to reconcile your belief in freedom and dignity for all people with those images of children writhing in pain, and going still on a cold hospital floor. For sometimes resolutions and statements of condemnation are simply not enough.

Obama was being charitable. While the Left has long been identified with the compassionate philosophy of pacifism, the debate over the past two weeks has shown a growing tendency on the Left to embrace the hard-hearted call of isolationism. If the isolationist trend continues, it not only makes Obama's immediate case for military intervention on humanitarian grounds a much harder sell, but over the long term it threatens the ideological underpinnings of liberalism itself.

A core tenet of liberalism is the belief that active government should take responsibility in alleviating or preventing the suffering of others. In turn, a liberal expressing pacifist opposition to a military strike in Syria would still accept responsibility for preventing genocide, and lobby for diplomatic and economic means to avert slaughter without risking the unintended consequences of violent force.

The Left's loudest spokesperson against a Syrian strike is Rep. Alan Grayson. His main argument is far colder: "This is not our problem."

Grayson's offers no alternative solutions, and instead frames a false choice between helping Syrians and helping Americans: "Our own needs in America are great, and they come first. The death of civilians is always regrettable, and civil war is regrettable, but no Americans have been attacked, and no American allies have been attacked." That petition language from Grayson has garnered more than 90,000 signatures so far.

Grayson even went as far as embracing Sarah Palin, telling radio host Ed Schultz, "I think I'm in agreement with Sarah Palin. She said, 'Let Allah sort it out.'"

It may seem shocking to see Grayson swimming in such bigoted waters. But this is where a flirtation with foreign policy libertarianism can lead. Once someone's else misery is "not your problem," it becomes ever easier to dehumanize an entire class of people and deem their lives, their homes, and their faiths to be worth less than yours.

Perhaps Grayson is shunning liberal principles in part for expediency. As legendary community organizer Saul Alinsky wrote in Rules For Radicals, "The price of a successful attack is a constructive alternative. Avoid being trapped by an opponent or an interviewer who says, 'Okay, what would you do?'"

By taking an isolationist line, you never have to answer that question. But such expediency comes at a cost.

Under President Obama, the Democratic Party has successfully swiped the national security mantle from the Republicans, precisely because he has repeatedly met politically risky foreign policy challenges instead of ducking them. Grayson, with plenty of poll numbers at his back showing little appetite for military strikes, is gleefully ducking the problem of offering constructive alternatives. That's all well and good as a congressional backbencher. Presidents, however, have to worry about not just poll numbers today, but also the effects of policies tomorrow. Democrats who want to retain control of the Oval Office need to have a bit more to offer than what Grayson is serving.

Furthermore, once you give up the essence of liberalism — empathy for others — you lose the underpinnings of all of your other liberal objectives. If suffering in someone else's country doesn't warrant our help, why should suffering in someone else's state or someone else's neighborhood?

Obama has two overarching reasons to beat back this growing isolationist sentiment: Save his foreign policy and ensure America does not regress toward conservatism once his term expires.

His speech on Tuesday sought to do so with a stiff dose of morality: "Terrible things happen across the globe, and it is beyond our means to right every wrong. But when, with modest effort and risk, we can stop children from being gassed to death, and thereby make our own children safer over the long run, I believe we should act. That's what makes America different. That's what makes us exceptional."

Those liberals who are steadfast against striking Syria are unlikely to be swayed. But they should stop to consider that as of this moment, Obama is making a moral case while their chief spokesman for rejecting military force is making an amoral case.

Obama's opponents on the Left, for the sake of liberalism, should reject Grayson's simplicity and callous bigotry, make clear that America and the world have a responsibility to do something, embark on the hard task of crafting serious alternative nonviolent proposals, and reframe the debate on the grounds of who has the better plan to prevent a Syrian genocide.

I don't know who would win that argument with the American people. But a debate on those terms would protect liberalism from succumbing to isolationism, and help ensure America doesn't quit striving to be a force for good in the world.