A Democratic candidate for the U.S. Senate joked with the male moderator of a televised debate that he was 'prettier' than Candy Crowley, the CNN who refereed this week's presidential debate.

After an intense exchange during Thursday’s Senate debate in Arizona, moderator Brahm Resnik quipped: 'Now I know how Candy Crowley felt, Jeez..'

Richard Carmona, former U.S. Surgeon General, responded to the reference to Crowley's difficulties officiating between Barack Obama and Mitt Romney by saying: 'You’re prettier than her.'

Prettier? Democrat candidate Richard Carmona had to apologise after saying Arizona senate debate moderator Brahm Resnik (left) was better looking than presidential debate moderator Candy Crowley (right)

Resnik filled the awkward silence by laughing and saying breezily: 'I'm not sure how to take that.'

Carmona, who was locked in a very tight race with Representative Jeff Flake, the Republican candidate, before the debate later apologised.

'I tried to tell a joke to lighten the mood in a debate,' he said in a brief statement to Politico. 'I shouldn't have, and I'm sorry.'

The moment could cause Carmona a lot of problems, not least becasue Flake has made an issue of allegations that Carmona has mistreated women.

Flake's campaign was already running an ad featuring Cristina Beato, a physician and former colleague of Carmona's, saying that the Democrat 'has issues with anger, with ethics and with women'.

Eyes for you: The remarks were made by Richard Carmona, left, during the debate with Republican Rep. Jeff Flake, right on Thursday

She recalls in the ad that Carmona once angrily pounded her partment door while he was serving as Surgeon General under President George W. Bush.

Carmona had responded with his own ad showing Kathleen Brennan, a retired police captain and his former SWAT team commander, saying: 'Rich treats everyone with respect. It doesn't matter whether you're male or female.'

Arizona, which is an open seat after the retirement of Senator Jon Kyl, a Republican, is a major Democratic hope for a gain. Flake, who had been expected to be elected relatively comfortably, is leading Carmona by an average of less than one per cent in the polls.

Carmona’s remarks were first flagged by BuzzFeed.