"Stop whining." Thus Joe Biden the other day, as the US Vice-President tried to fire up the flagging Democratic faithful for the midterm elections, which are now barely four weeks away and promise nothing but trouble for the party.

Older readers may recall that a White House campaign by America's current VP once foundered on charges of plagiarising a certain Neil Kinnock – and even older ones will remember him as the windiest senator of them all. But these days, as he runs through the plagues Republicans will visit on the land should the enemy take control of Congress, Biden is even being quoted by his boss. "As Joe says," Barack Obama noted in a speech last week, "don't compare us to the Almighty; compare us to the alternative."

Alas, Democrats these days need a lot of firing up. The standard wisdom of this campaign is that the party's biggest problem is a reinvigorated Republican Party, goaded on by ultra-conservative Tea Partiers, that has become the vehicle for the country's anger and frustration. But the real difficulty faced by Messrs Biden and Obama is the mirror image – the disappointment and apathy of the liberal Democratic base, whose enthusiasm swept them to power just 23 months ago.

At the White House itself, the mood is one of endings, not beginnings. As expected, Rahm Emanuel, the White House chief of staff, officially took his leave on Friday at an East Room ceremony in search of a quieter life as mayor of Chicago. A clutch of other senior advisers are also on the way out. Almost certainly, the great legislative achievements are over; the rest of Obama's term will be a war of attrition with resurgent Republicans, as the focus switches to his own re-election effort in 2012.

The President and Vice-President are doing their utmost to rekindle the flame. Obama is back on the stump, mixing "hear-the-people's-grievances" meetings in supporters' back gardens with the giant rockstar rallies of campaign 2008 when, like no presidential candidate before him, he caught the imagination of the young.

But even at an event last Tuesday on the famously liberal campus of the University of Wisconsin in the state capital, Madison, in front of an overflow crowd of 25,000, the magic was missing. It might have been a glittering early autumn day in the upper Midwest. But in political terms, the bright dawn of November 2008, when anything seemed possible in America, has turned into a dreary, drizzly afternoon when nothing can get done. And many liberals feel Obama has let them down – that he hasn't fought for what he believed.

In a sense the charges are unfair. No one would deny that the 44th President was dealt a dreadful hand: two wars, a financial system close to melt-down, huge deficits and the worst economic slump since the Great Depression. You may blame, too, the sheer depth of the recession, and the excessive faith placed by ordinary citizens in a President's ability to work wonders. The wars and the lousy economy may have originated under George Bush, but Americans have short memories. Bush is ancient history. These are Obama's wars and Obama's economy; it's his fault things aren't back to normal.

Like all Democrats, liberals point the finger of blame at the intransigence of the Republicans, made easier by a dysfunctional political system, above all in the Senate where the 60-vote majority required to overcome a filibuster enshrines a tyranny of the minority. Politics, after all, is the art of the possible, and to achieve more than Obama achieved may well have been simply impossible.

Liberals look back and think what might have been. Take healthcare reform. A step in the right direction, they admit – and they further acknowledge that a single-payer system along European lines was a non-starter. But why did Obama not fight harder for a "public option", a government-run scheme that would have undercut the rapacious insurance companies?

You hear similar arguments about the financial regulatory bill that is supposed to ensure a 2008-style crash never happens again. The measure might have cut Wall Street down to size. Instead, liberals complain, the lobbyists were allowed to water it down, banks can still gamble in the markets on their own account, and bonuses are back to pre-bust levels.

What stings most, however, is Obama's readiness to preserve controversial Bush-era programmes, of dubious legality, in the "war on terror". Guantanamo Bay, we were told, would close by January 2010. It is still very much in business, and the military tribunals to try prisoners are as secretive as ever. Some of them may face indefinite detention without trial, which is against every precept of international law. The worst excesses of the CIA are supposed to have been abolished – but who can say for sure?

Meanwhile the use of unmanned attack drones has, if anything, grown in countries such as Yemen and Pakistan where the US is not formally at war; for many on the left, the strikes amount to illegal assassination. Worst of all they suspect that Obama is guided primarily by a desire to rebut Republican charges that Democrats are wimps on national security.

In this respect, the decision to throw more troops at an unending, probably unwinnable war in Afghanistan is Exhibit A. Bob Woodward's new book (of which I wrote in this space a week ago) has been especially damaging. Its account of the tortured decision to launch the surge suggests Obama does not believe in his own policies but, even more importantly, did not want to be perceived as weak.

The President now berates "griping and groaning" Democrats. "Folks, wake up," he tells his suppporters. But it may be too late. Yes, disillusioned liberals have nowhere else to go; but they provide the grassroots enthusiasm vital for the party's success. Had Obama on occasion gone for broke, and suffered a few heroic failures, they might be fighting harder now. But the lesson of the past two years is that he is a cautious man and a realist. He may soon be paying the price.