A US by-election tomorrow to replace a jailed Congressman in a traditionally safe Republican seat is too close to call, an alarming sign for President Bush and his party ahead of November’s mid-term elections.

Republicans fear that a Democrat victory in the unexpectedly close contest to replace Randy “Duke” Cunningham, the disgraced Congressman who pleaded guilty last year to accepting $2.4 million (£1.3 million) in bribes, could signal a national trend against them and the loss of Republican control on Capitol Hill in November.

Despite Cunningham’s spectacular downfall, his House of Representatives seat — California’s 50th district — has for years been so solidly Republican that even the most bullish Democrats initially believed that they had no chance of capturing it when he resigned in November. But last night, in a race being closely watched by both parties, Francine Busby, the Democrat candidate who lost to Cunningham’ s landslide in 2004, was in a statistical tie with her Republican opponent, Brian Bilbray, a former congressman turned lobbyist.

Republicans have become so alarmed over the possibility of losing the seat that last week Dick Cheney, the Vice-President, flew across the country to campaign for Mr Busby. “Right now the eyes of the nation are on the 50th district of California,” Mr Cheney declared.

The national Republican party has spent $4 million on the race, an enormous sum considering the district’s demographics. In 2004 Cunningham won 58 per cent of the vote to Ms Busby’s 36 per cent, while Mr Bush won 55 per cent in the presidential contest.

“A Busby win would be a huge triumph for Democrats, and probably presage a Democratic takeover of the House in November,” said David Frum, a former White House speechwriter for Mr Bush.

Ms Busby has run her campaign on national themes of Republican sleaze, a thirst for change among voters, and in the light of the Jack Abramoff scandal, refers to her opponent as “lobbyist Bilbray”.

The centrepiece of Mr Bilbray’s campaign has been a hardline stance on immigration. The district, in San Diego’s most affluent neighbourhoods, is just 20 miles north of the Mexican border.

Mr Bilbray contends that the race does not have national implications. He points out that Mr Cunningham’s fall from grace, at a time when Mr Bush’s approval ratings are at record lows and the Iraq war grinds on, changed the dynamics of the contest.

But privately Republicans say that the result could signal how disaffected voters are with Mr Bush and Republicans generally, and also how the issues of sleaze and immigration will affect voting trends.

In California tomorrow and nationally in November, Republicans must turn out their conservative base.

To that end, Mr Bush will today make a speech calling for a Constitutional ban on gay marriage, a proposal that has no legislative chance of success but is aimed at rousing disaffected conservatives.

Arnold Schwarzenegger, the Governor of California, will discover tomorrow who his Democrat opponent will be when he seeks re-election in November after a Democrat primary contest between Steve Westly, State Controller, and Phil Angelides, State Treasurer.

Times Recommends

• Henning Mankell: ‘If Wallander was alive, I wouldn’t be a friend of his’
• Secret society's skull and crossbones for sale
• Nigerian leader vanishes after medical treatment