[1] Rangsiman gained national-level visibility as a political activist in the aftermath of the 2014 Thai coup d'état, and was a member of the New Democracy Movement.

[3] On 15 February 2023, during a parliamentary debate regarding the government's performance on tackling illicit drugs and the shadow economy, Rangsiman accused Prayut Chan-o-cha of associating with corrupt figures and neglecting the government's pledge to address drug issues, He implicated senator Upakit Pachariyangkun in a money laundering and drug trafficking case involving Upakit's son-in-law and Tun Min Latt, a Burmese businessman.

[4][5] At a press conference thereafter, Rangsit accused Upakit of falsely declaring his assets before assuming his senate seat in 2019.

[5] During the debate, Rangsiman also exposed the issue of corruption within Thai law enforcement agencies in relation to an alleged Chinese triad leader.

[6][7] On February 19, Thai police announced they would press charges against the individual in question, a Chinese national.