In 1981, unhappy with the influence of the more left-wing members of the Labour Party, she resigned her membership to form – along with fellow Labour resignees Roy Jenkins, David Owen and Bill Rodgers – the Social Democratic Party (SDP). They were joined by 28 other Labour MPs and one Conservative. Later that year, following the death of the Conservative MP Sir Graham Page, she won the Crosby by-election and became the first SDP member elected to Parliament. Two years later, however, having become the SDP's President, she lost the seat at the 1983 general election. At the 1987 general election, Williams stood for the SDP in Cambridge, but lost to the sitting Conservative candidate Robert Rhodes James. She then supported the SDP's merger with the Liberal Party that formed the Liberal Democrats.
In 1981, unhappy with the influence of the more left-wing members of the Labour Party, she resigned her membership to form – along with fellow Labour resignees Roy Jenkins David Owen and Bill Rodgers – the Social Democratic Party (SDP). They were joined by 28 other Labour MPs and one Conservative. Later that year, following the death of the Conservative MP Sir Graham Page, she won the Crosby by-election and became the first SDP member elected to Parliament. Two years later, however, having become the SDP's President, she lost the seat at the 1983 general election At the 1987 general election, Williams stood for the SDP in Cambridge but lost to the sitting Conservative candidate Robert Rhodes James. She then supported the SDP's merger with the Liberal Party that formed the Liberal Democrats.