Baroness Shirley Williams said fellow Liberal Democrat peer Lord Rennard, who is currently being investigated for allegations of sexual harassment, is "absolutely selfless" and at worst guilty of "misbehaving".
In an interview with parliamentary magazine The House, the Liberal Democrat grandee also said women would expect "people to take advantage of you" when she entered politics in the 1960s.
The allegations concerning Lord Rennard, one-time chief executive of the party, came to light in a Channel 4 news report earlier this year. He denies any wrongdoing.
But Baroness Williams said in the interview: "We owe an awful lot to Chris in terms of the work he has done for the party. He has been absolutely selfless."
She emphasised she did not know whether there was any truth in the allegations but said groping should generally be "warned against" and "the person concerned should apologise and stop misbehaving".
On changes in attitudes towards sexual assault over the decades, Baroness Williams, who has been in Parliament on-and-off for nearly 50 years, said: "On the whole, the general view taken was that if you were a young woman in politics, well what did you expect if you were trying to get to somewhere near the top? People would take advantage of you.
"One famous cabinet minister used to chase you round the filing cabinets which was a lot worse than a hand on your knee. It's changed slowly, but we have got somewhat obsessed about getting very exaggerated reactions to what is silly and impolite and discourteous behaviour. But it's not the same as violent or intimidating behaviour."