BANGKOK (AP) — Thailand on Monday officially began the selection of new senators, a process that has become part of an ongoing war between progressive forces hoping for democratic political reforms and conservatives seeking to keep the status quo.

Hopeful candidates headed to district offices across the country on the first day of registration to compete for one of the 200 seats in Parliament’s upper house.

The power of the Senate — although limited compared to the House of Representatives, which is tasked with law-making responsibilities — was demonstrated dramatically when it blocked the progressive party that won the most seats in last year’s election from forming a new government.

The senators were able to do so because of the 2017 Constitution, passed under a military government, which requires the prime minister to be approved by a joint vote of the elected House and the Senate, which was appointed by the military regime.