A spokesperson for expected Republican presidential candidate Mitt Romney has resigned, after critics questioned his conservatism because of his sexuality.

Richard Grenell, who is gay, announced on Tuesday that he has decided to leave Romney's campaign after just a few weeks.

In a statement, Grenell said his "ability to speak clearly and forcefully on the issues" was hurt by what he called "hyperpartisan discussion of personal issues".

Grenell previously worked for John Bolton, Washington's former ambassador to the UN, as well as other foreign policy hawks.

Grenell's sexuality has caused right-wing critics to raise questions about his conservative credentials.

The departure underscores some of the challenges the Romney campaign faces as it tries to broaden its appeal to more moderate voters without alienating the social conservatives who make up the party's base.

Matt Rhoades, Romney's campaign manager, said the team was disappointed with Grenell's decision, saying he "had superior qualifications for the position he was hired to fill".

Before he was hired to join the campaign, Grenell was criticised for what were deemed as rude remarks he made on Twitter about prominent women, including Hillary Clinton.

"Hillary is starting to look like Madeleine Albright," Grenell tweeted recently about the US secretary of state, comparing her to the first woman to hold the job after being appointed by Bill Clinton in the 1990s.

Grenell's tweets were deleted when he started to work with the Romney campaign.