LONDON — Prime Minister David Cameron is usually the nimblest of politicians, radiating self-assurance and blessed with an almost Reaganesque ability to deflect criticism. But as the phone hacking scandal spreads, Mr. Cameron has been placed in the unaccustomed position of appearing vulnerable and behind the curve.

He has been maneuvered into embarrassing U-turns nearly every step of the way, and on Tuesday performed the latest one: suddenly joining the opposition Labour Party, his bitterest foes, in calling for Rupert Murdoch’s News Corporation to withdraw its $12 billion bid to buy British Sky Broadcasting, known also as BSkyB.

Mr. Cameron’s opponents in turn have seized on the chance to inflict damage on the once-unassailable prime minister. The scandal has given new life to the Labour Party and its leader, Ed Miliband, and there are signs, too, of cracks in the governing coalition between the Conservatives and their until-now toothless Liberal Democrat partners. The Liberal Democrat leader, Deputy Prime Minister Nick Clegg, on Monday asserted a new independence by heaping moral outrage on Mr. Murdoch and urging him to rescind his BSkyB bid — a full day before Mr. Cameron would do the same.

But the prime minister’s problems go deeper than failing to read the political signs as quickly as other parties. More seriously, his critics say, the affair raises questions about Mr. Cameron’s character and judgment in cultivating multiple ties to News International, Mr. Murdoch’s British subsidiary, which helped put him in office but which is currently about as politically popular as a basket of snakes at a summer picnic.