"UDF's assumption is that Congress MLA Vincent, against whom the charge has been made, has not done anything wrong and the complaint of the woman in this regard is false," Front Convener P P Thankachan told reporters after a meeting of the alliance's leaders here.
The United Democratic Front also said it was dissatisfied over the present probe and asked the government to conduct an impartial investigation.
Thankachan alleged that the CPI(M)-led LDF government had "misused" the Women Protection Act to "frame" the legislator.
Thankachan said some front leaders also alleged at the meeting that Chief Minister Pinarayi Vijayan's office had intervened in the case.
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Congress suspended him from the party secretary post on July 23, but defended him saying "the case and his arrest are politically motivated".
Vincent has been accused of raping a 51-year-old woman, who attempted to commit suicide by overdosing on sleeping pills.
After a complaint from the woman's husband, the police questioned the Congress MLA for over three hours at the MLA hostel on July 22 after which he was arrested.