A poll last week found that voters, by 50% to 37%, would prefer the Democrats to win control of Congress. If Bush’s opponents find themselves in a position of power, the temptation to humiliate him is likely to be irresistible.

A taste of the battle to come was provided last week by Senator Russ Feingold, a popular choice for the 2008 presidential nomination among Democrat anti-war activists. He proposed a motion of censure against Bush for authorising the National Security Agency to wiretap Americans suspected of links to terrorism without a court warrant.

Feingold gave notice that the party should stop “cowering” before Bush on national security issues. “If there’s any Democrat out there who can’t say the president has no right to make up his own laws, I don’t know if that Democrat really is the right (presidential) candidate,” he said.

Plenty of his Senate colleagues ducked for cover, fearful of alienating either party activists or swing voters. The eavesdropping issue is one of the few hot-button topics where Bush has public support. One leading supporter of Hillary Clinton acknowledged ruefully: “It’s hard to beat the argument, ‘If Al-Qaeda is on the line, we want to be listening’.”

Clinton, the party’s presidential frontrunner, hid last week from reporters who wanted to question her on the censure motion while she was attending a Democrat lunch at the Senate. Most Democrats would rather keep their options open on impeachment than pronounce one way or another.

Republicans are practically begging them to “bring it on” in the hope that the chatter will tar their opponents as loony leftists who care nothing for national security. “This is such a gift,” said Rush Limbaugh, the right-wing radio chat show host.

For demoralised conservatives, the issue is a call to arms for the mid-term congressional elections. “Impeachment, coming your way if there are changes in who controls the House right now,” Paul Weyrich, a top conservative organiser, warned in an e-mail newsletter to supporters.

“With impeachment on the horizon maybe, just maybe, conservatives would not stay at home after all,” Weyrich wrote.

If few senior Democrats are calling publicly for Bush to be placed in the dock, plenty are flirting with the idea. One of them is Al Gore, the defeated 2000 presidential candidate, who is increasingly talked up as a serious anti-war contender at the next election.

Gore said recently that Bush’s “unlawful” eavesdropping was part of a larger pattern of “seeming indifference” to the American constitution, which could well be an impeachable offence.

John Kerry, the 2004 presidential nominee, was overheard in an Irish bar on Capitol Hill talking about how satisfying it would be to impeach Bush if Congress went Democrat. He was just having a laugh, his spokeswoman rushed to explain: “Impeachment jokes in Washington are as old as Donald Rumsfeld.”

But then she turned serious: “How are the same Republicans, who tried to impeach a president over whether he misled a nation about an affair, going to pretend it does not matter if the administration intentionally misled the country into war?”

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