Politics can be a dirty business. But when, like Barack Obama, you are running a campaign that seeks to change the tone of politics, fighting back presents its own difficulties.

On Friday Hillary Clinton’s campaign lost no time in seizing on an off-the-record comment by Samantha Power, a senior foreign policy adviser to Mr Obama, in which she described Mrs Clinton as a “monster”.

The comment, which was made to The Scotsman newspaper in the UK, was nevertheless published. It was followed swiftly by Ms Power’s resignation. “I made inexcusable remarks that are at marked variance from the spirit, tenor and purpose of the Obama campaign,” she said. “I extend my deepest apologies to Senator Clinton and Senator Obama.”

The incident threw into sharp relief the key dilemma of Mr Obama’s campaign: how can he retaliate against an onslaught from Mrs Clinton without jeopardising his crowd-pulling reputation for being above it all? The urgency of this question coincides with concern among Mr Obama’s supporters that he appears to back off when the going gets tough.

Instead of showing defiance as Mrs Clinton has done on nights when she has lost elections, Mr Obama’s speech in San Antonio, Texas, on Tuesday night struck many Democratic observers as too resigned and detached.

Mr Obama’s defeat followed a torrid week in which Mrs Clinton threw “the kitchen sink” at him in negative political advertisements and co-ordinated attacks on his integrity. 

“It is a legitimate question to ask whether Barack Obama is tough enough to fight Mrs Clinton because he would get a lot worse from the Republicans, were he the nominee,” says Tom Schaller, political scientist at the University of Maryland.

Among Democrats the question is particularly relevant, given the costly delays that John Kerry, the 2004 presidential candidate, permitted in response to the infamous “Swift Boat” advertisements that questioned his Vietnam war record. Many believe that Mr Kerry’s prevarications cost him the White House.

With almost seven weeks to go before the next big primary election in Pennsylvania, Mr Obama this week signalled he would start to take a much tougher line against Mrs Clinton. On Wednesday his campaign sent out a hard-worded memo calling on Mrs Clinton to release her tax returns and to accelerate publication of the presidential records concerning her period as first lady.

At the same time the Clinton campaign has stepped up demands on Mr Obama to reveal more about his relationship with Tony Rezko, the Chicago real estate developer, whose trial on corruption charges started on Monday.

Mr Obama’s recent string of bad news may well end tonight when results are declared from the caucus elections in Wyoming, where he is polling well ahead of Mrs Clinton. He is also thought likely to win the primary election next Tuesday in Mississippi, given its large share of African-American voters. The real test, however, will come in Pennsylvania on April 22.