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US Vice-President Joe Biden has attacked Democratic presidential candidate Hillary Clinton for saying that Republicans are her enemies.

Mr Biden told an event in Washington it was impossible to govern without co-operating with political opponents.

Speculation has mounted this week that the vice-president is about to join the Democratic race for the White House.

If he does, he will have to overcome the current frontrunner and former Secretary of State Mrs Clinton.

She was asked last week in a Democratic debate which enemy she was most proud of making.

"Well, in addition to the NRA, the health insurance companies, the drug companies, the Iranians; probably the Republicans," she said.

Three times this week, Mr Biden has tried to distance himself from her statement, starting with a climate change speech on Monday in which he said Republicans were not enemies but friends.

And on Tuesday, he repeated the point twice more, adding, with a trace of exasperation: "End this notion that it's naive to think we can speak well of the other party and co-operate.

"What is naive is to think it's remotely possible to govern this country unless we can."

He has also said this week that he privately urged the president to go ahead with the operation to kill Osama Bin Laden, adding to a sense he could be preparing to run.

The Democratic and Republican primary contests begin in February, and voters will go to the polls next November.

Analysis - Anthony Zurcher, BBC North America reporter

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Joe Biden is making subtle - and not-so-subtle - digs at his party's frontrunner. He's reportedly seeding sympathetic stories in the press. Supporters are talking about the campaign infrastructure they will build once they get the go-ahead.

These are the kind of moves that have stoked speculation that Mr Biden is gearing up to take on Hillary Clinton - and, at least for the moment, Mr Biden is doing little to put out the fires.

And yet hours, days, weeks pass, and the vice-president has yet to make his decision. It's enough to prompt speculation that the could-be candidate is simply enjoying his time in the spotlight before closing the door on a third presidential bid.

Mr Biden may end up staying on the sidelines, but he continues to act like a man who wants to be president. Although he many never become a candidate, he has spent a lot of time recently sounding like one.