"I wish to put the record straight that the government led by Prime Minister Yingluck Shinawatra has been democratically elected by the Thai people and gained the majority in the House of Parliament in accordance with the democratic system," wrote Noppadon Pattama.
Mr Noppadon, also a former foreign minister, was moved to write to Barack Obama after Mr Suthep penned his own defence of the People's Democratic Reform Movement (PDRC) for the US president's benefit. Mr Suthep claimed Mr Obama was getting misinformation about Thai politics from some western media.
Mr Noppadon said Mr Suthep was not being entirely straight about his group's political goals when he called for reform.
"The reform is just a justification for the PDRC to acquire power to govern Thailand without going through an election," he said, adding that the PDRC was violating the rights of voters.
"While democracy and the Constitution are fundamental guarantees of rights and freedoms of American citizens, they also are for Thai citizens," his letter stated.
Mr Suthep has said the PDRC would boycott the election on Feb 2 but stressed that it did not oppose a poll. An election can be held after reforms are completed, he added.