Senator Joseph Lieberman, a stalwart backer of John McCain who calls himself an "independent Democrat", could face punishment from the Democratic party if he is asked to speak at this summer's Republican convention.

Lieberman has said he would appear at McCain's nominating convention if invited by the Republican, fuelling frustration among US liberals already angered by his support for the Iraq war. Democrats revoked his superdelegate status earlier this year.

The onetime vice presidential nominee of Al Gore has played a pivotal role in maintaining Democrats' narrow control of the Senate. But party leader Harry Reid today left the door open to sanctioning Lieberman if he speaks in favour of McCain's nomination.

"I'll consider anything," Reid said at a breakfast with reporters today, adding that the Democratic presidential nominee, likely Barack Obama, would want to weigh in on Lieberman's future role.

Reid noted that Lieberman's reliable alignment with Democrats on domestic issues such as education and taxes - he has proposed a "war on terrorism" tax to fund the US presence in Iraq - makes the case for his continued value in the party.

The Connecticut senator sides with Democrats "90 to 95%" of the time outside of war-related issues, Reid said.

But Iraq has permanently discoloured Lieberman's standing with some US Democrats who believe his praise for George Bush has undercut the party's efforts to end the war. The strong bond between Lieberman and McCain has only exacerbated those doubts.

Democrats nearly faced a similar conundrum in 2004 when their senator from Georgia, Zell Miller, publicly denounced presidential nominee John Kerry at the Republican convention. Miller's previously announced retirement from politics that year spared him sanctions within the party.

The possibility of punitive action against Lieberman exists largely because of Democrats' excellent position in congressional elections this fall. Reid said the party is well positioned in as many as 11 Senate races, including several in traditional Republican bastions such as Alaska and Mississippi.

"We're going to pick up seats," Reid predicted.

The majority leader also addressed the future role of another influential senator: Hillary Clinton, whose presidential campaign is fading out as Obama builds on his lead in the delegate count.

The prospect of a former first lady and near-nominee returning to the Congress without a leadership post has led to speculation that Clinton might seek a senior role under Reid - or even contest for Reid's job.

Yet Reid quelled any speculation that he would make way for Clinton or that she would snag a new position upon her return.

"A senator coming back who ran for president, that's not unique around here," Reid said, referring to Kerry and two other senators who lost presidential bids, Joseph Biden and Chris Dodd.

"Senator Clinton has some very fine committee assignments. She's going to have a lot to do."