Former party leader Charles Kennedy vowed to give up alcohol on a number of occasions but lapsed when under pressure, a senior Lib Dem has said.
Mr Kennedy resigned after admitting to a drink problem
Shirley Williams, ex-Lib Dem leader in the House of Lords, told BBC Radio 4's Desert Island Discs Mr Kennedy had made a "real effort" to give up drinking.
But he did not realise how hard it was to break the habit, she said.
She said Mr Kennedy's resignation and that of party home affairs spokesman Mark Oaten had been a setback.
Mr Oaten resigned last weekend over claims he had an affair with a male prostitute.
Lady Williams said the party would take a long time to recover from the two departures.
On Mr Kennedy, she said he had "promised more than once to give it [alcohol] up" and made "real efforts", but eventually "under pressure, lapsed".
"I think the problem is that many people with the issue of alcoholism as a sort of disease do tend to believe that they can break the habit and it is much, much harder than they believe," she said.
"I did know a bit about Charles' problem and one has to say that alcoholism or heavy drinking is an occupational hazard of politics.
"I think it would be fair to say that someone like Winston Churchill more or less marinated in alcohol and it didn't seem to affect his performance.
"Somebody on the other hand like George Brown (Labour deputy prime minister in the early 1960s), who I knew well, was driven to extremes by just a couple of sherries."
But Lady Williams said she was upset rather than resentful over the setbacks facing the party.
"It's very sad and I think one of the harsh things.... is that by and large the Liberal Democrats were held, and to some extent held themselves, to a higher standard, and that's where part of the pain comes."
Lady Williams left Labour in 1981 to form the SDP, and in 1988 supported the merger with the Liberals which formed the Liberal Democrats.
The full interview will be aired on Radio 4 at 1115 GMT on Sunday.