Activists at last week’s Netroots Nation talked about disappointment and disillusionment. The polls show a slow, steady decline in support for the president among Democrats. Neither sample captures perfectly the state of the liberal mind this summer, but you’d have to be pretty oblivious not to see that President Obama, and the Democrats, are losing the love of their base.

It’s a somewhat predictable decline, given lofty expectations for the Obama presidency and the stubbornly slow recovery. It's also a relatively modest decline: After all, it’s not like anybody is talking about starting a third party. Still, the right is energized, the left is ambivalent, and that means Democrats are in big trouble this November.

If you read this blog, then you know I see things more or less the way my colleague Jonathan Chait and some of our friends in the blogosphere do: This seems totally nuts, purely on the merits. Obama and the Democrats passed a major stimulus that cut taxes for the middle class and invested heavily in public works. They saved the auto industry, created a new regulatory framework for the financial industry, and enacted comprehensive health care reform. Compromises watered down each of these initiatives, to say nothing of the ideas (climate change!) that aren’t going to pass. And still this was the most productive liberal presidency in a generation or maybe two.

But liberal ambivalence isn't just foolish substantively. It's also foolish strategically.

The fact is that voting for these measures, particularly health care and (in the House) climate change, was tough for many members of Congress. Liberals consider the Affordable Care Act a watered-down version of a watered-down of something resembling a true universal coverage system. But in Tennessee, Idaho, and a bunch of places in between, it's a government takeover of health care. Liberals think Waxman-Markey was a conservative half-solution to a planetary crisis. In more conservative districts--and, let's face it, plenty of liberal ones too--it's higher energy bills.

But consider what happened after the climate change vote in the House last year. When Democrats went back to their districts, conservatives pummeled them--in person and on the air--while liberals just shrugged. And consider  what happened after the health care bill passed: Conservatives went into overdrive about socialized medicine, while liberals kept talking about what a lousy bill it was.

Not surprisingly, members from more conservative parts of the country are pretty frustrated, particularly when they're getting attacked directly by the left. As one senior Democratic aide told me on Wednesday, expressing a sentiment I've heard many times on Capitol Hill,

Liberals have savaged these members and the lesson many will take is don’t stick your neck out because the left will kick your ass regardless.

To be clear, sometimes ass-kicking is good. Call Kent Conrad a hypocrite on the deficit. Blast Joe Lieberman for carrying water on behalf of the insurance industry. Hold Obama accountable for the bureaucratic neglect that enabled the Gulf disaster. Liberals won't get anywhere by meekly accepting every compromise that comes down the pike or looking the other way when Democrats screw up. Politics goes is a two-way street and liberals need their leaders to lead sometimes.

But if the left is going to demand action, it has to do more than sigh when action--even modest action--actually happens. The left has to show some enthusiasm, if not locally then at least nationally. (Truth be told, a Democratic member in a Republican district probably benefits more from higher Obama approval ratings than an ad buy from Moveon.org). Otherwise office-holders, even ones from relatively liberal districts, won't have much incentive to vote liberal next time around. As another congressional aide put it, via email:

I hear this stuff all the time, about climate change, health reform, financial reform--members complaining about having to vote for these things because they were forced to by party leadership with NO upside for them. ... They’re getting hit on all sides. ... these members need more than just the stick, you also have to give them the carrot every once in a while.

It'd be nice if we lived in a world where politicians voted exclusively based on the public interest. But we happen to live in a world where, to varying degrees, politicians vote based on their immediate electoral needs. If liberals don't embrace politicians who vote with them today, then liberals can't expect the same politicians, or their replacements, to vote with them tomorrow.

Update: I tweaked some language for clarity