By Daniel Griffiths BBC News, Washington

The gay marriage debate could impact on the election

And they are all the more surprising because the vice-president has a reputation for being a tough conservative, closely allied to President Bush.

But he also has a daughter who is openly gay and who works on the Republican election team.

Traditionally, he has not spoken publicly about his children and rarely comments on social matters such as this one.

But Mr Cheney has clearly struggled with an issue which for him is both political and personal. Hence his words while on the campaign trail in Iowa.

He said people ought to be free to enter into any kind of relationship they wanted.

And he said individual states, and not Washington, should be allowed to rule on the issue of homosexual marriage.

Collision course?

These are explosive comments which seem to fly directly in the face of President Bush.

The Cheneys take "enormous pride" in gay daughter Mary

Earlier this year the president backed a federal ban on gay marriage, something that Vice-President Cheney reluctantly supported.

The Bush administration had used the issue to bolster support among conservative groups.

They make up an important Republican voting bloc and oppose any expansion of gay rights - and now they are hopping mad.

Genevieve Wood of the Family Research Council said: "The vice-president's remarks are disappointing. The fact is I think it sends a very mixed message to voters. Where does the administration stand on this issue?"

Gay Republicans

But gay rights activists have welcomed the vice-president's words.

"It was a break with the president and it reminds all of us why the Republican party ought not to divide this country," said Patrick Guerreiro of the lobby group Log Cabin Republicans.

Dick Cheney says his views are personal and that they have no effect on White House policy.

But there is no doubt that his comments strike at the very heart of President Bush's thinking on this issue.

Topping the agenda

And they come at a time when gay marriage is shaping up to be an extremely controversial debate in this year's presidential election race.

Earlier this year Massachusetts Supreme Court ruled that homosexual couples had the right to marry.

But public opinion in most states is against gay marriage.

Voters in Missouri this summer chose to ban it. And many other states are expected to decide on the issue in the autumn.

All that puts the issue firmly in the public eye and the row is likely to spill over into the Republican Convention which begins on Monday.

Party strains

Conservatives are pushing for a constitutional ban on gay marriage to be part of the party's platform.

That has angered moderates who favour a more relaxed approach.

It is the thing the organisers want to avoid - a fight which will highlight big divisions within the Republican Party.