There was speculation on Tuesday that the next to join an exodus ahead of the November elections could be Blanche Lincoln, who represents the conservative southern state of Arkansas and is behind every putative Republican challenger in opinon polls.

The Senate Majority Leader, Harry Reid, is meanwhile trailing all his potential Republican opponents in his state of Nevada, and even Mr Obama's old Senate seat in Illinois is expected to be close run.

The White House was rocked on Monday by the announcement by Evan Bayh, a popular, centrist senator from Indiana, who became the fifth senator to confirm he will not run for re-election. The latest speculation of more senators coming forward will only add to President Obama's woes and raise the prospect his party could lose its majority in Congress.

The emerging consensus in Washington is now that the Democrats have only a 50-50 chance of keeping control of the Senate, where they currently hold 59 out of the 100 seats, in what would be a stunning reversal of fortune after the party's clean sweep in 2008.

It has already lost the 60-strong majority that automatically overrode procedural blocks, after little known Republican Scott Brown last month captured a Massachusetts seat held by the late Edward Kennedy for 47 years.

Other Democrats are expected to withdraw from fray before deadlines fall for standing in the midterms, when 33 Senate seats and all 435 House of Representative seats will be contested.

The ruling party's vulnerability was further underlined on Tuesday when Frank Lautenberg, the 86-year-old senator for New Jersey, collapsed at his home and was hospitalised for the treatment of an ulcer.News of his collapse served as a reminder of the frail health of other senators, such as Robert Byrd, at 92 the oldest and longest serving member of the Senate, and Arlen Specter, an 80-year-old cancer and brain tumour survivor who faces a gruelling re-election battle in Pennsylvania.

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