Grassroots activists whose energy and donations have helped to propel Barack Obama towards the White House are suddenly choking on the bitter pill of disillusion.

In less than a month since clinching the Democratic nomination, he has performed a series of policy pirouettes to assuage concerns about his candidacy among a wider and more conservative electorate.

It is change, but not the type for which many of those who enthusiastically supported Mr Obama during the primaries had hoped.

The biggest group on Mr Obama's own web portal was one pleading with him yesterday to vote against domestic wire-tapping of terror suspects, which gives phone companies immunity from prosecution for past misdeeds.

By 11am 18,733 activists had joined the group, a fivefold increase in a week since he pledged support for the Bill. Previously he had described this measure as violating basic civil liberties, adding: “We have to make clear the lines that cannot be crossed.” On Thursday, Mr Obama posted his own message on the site, saying he was “happy to take my lumps” because democracy could not exist without dissent. While some people may view his position as a deal-breaker, he said, “our agreement on the vast majority of issues that matter outweighs the differences”.

By then, though, he was fighting another fire over remarks he made in North Dakota suggesting that he would “refine” his policy of a withdrawal of all combat troops from Iraq within 16 months of taking office.

He later hastily convened a second press conference to insist that he was not “searching for manoeuvring room” because he had always said he would listen to commanders on the ground. Mr Obama's emphasis on examining whether hard-won improvements in security had changed the conditions for a pull-out, contrasts to what his campaign manager said only a few months ago in response to earlier hints of a rethink. Back then, David Plouffe had stressed that the plan was “a rock-solid commitment ... there should be no confusion about that”.

On Mr Obama's website yesterday, Dianne from Detroit was typical of hundreds of other message senders as she warned him that he was making the same mistake as John Kerry in 2004 by “trying to be all things to all people”. She said: “First it was the telecom industry, now it's backing away from the timetable to withdraw the troops ... he will lose progressive Democrats if this continues.”

Internet activists, the so-called “Net-roots”, are similarly anguished over Mr Obama's praise for a Supreme Court decision striking down a ban on handguns and his nuanced criticism of another ruling against the death penalty being used for child rapists.

He has also softened his opposition to free trade deals such as Nafta, hardened his line against Iran and condemned a liberal group that branded General David Petraeus, the US commander in Iraq, as “General Betrayus”.

After a week in which he has campaigned on the themes of patriotism, faith and veterans' rights, Mr Obama was at it again yesterday.

He told a Christian magazine that pregnant women suffering “mental distress” should not qualify for late-term abortions, a distinction that sets him apart from most pro-choice advocates.

Mr Obama's aides say that he has always been a centrist politician seeking to unite America rather than divide it.

They suggest that some of the disappointment being felt is a consequence of the Left “projecting their own values” on to him.

Some groups such as Democrats.com are now telling supporters to pay money into special “escrow funds” that will be available for Mr Obama's campaign only if he lives up to his promises.

Latest poll

43% McCain
48% Obama

Source: Rasmussen poll, July 3

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