The polling began at 8 am and the voting percentage till 1 pm was 41.37 per cent, according to the Election Commission's website.
51,687 polling booths have been set up in 199 out of the total 200 assembly constituencies where polling is going on amid tight security.
A senior police officer said polling was going on peacefully and elaborate security arrangements are in place.
The results will be out on December 11, along with those from the other four states which saw Assembly elections in the past few weeks.
Manvendra Singh switched to the Congress just before the election, making the fight tougher for Raje this time. She had won 63 per cent of the votes cast in 2013, winning the seat by a margin of 60,896.
Tonk, with a sizeable Muslim population, is a keenly-watched contest between Sachin Pilot and BJP candidate and Rajasthan Transport Minister Yoonus Khan, who is the saffron party's only Muslim face in the elections.
This is a maiden assembly election for Pilot, a two-time MP who is seen as a chief ministerial possibility if the Congress wins. He has represented Dausa and Ajmer Lok Sabha constituencies in the past.
In about 130 constituencies, the contest appears to be mainly between the BJP and the Congress.
In the current House, the BJP has 160 seats and the Congress 25.