“As for Prime Minister Prayut’s chances of winning the election, they are, I would say, increasingly coming into question as the latest polls seem to indicate that there’s going to be a strong showing from Pheu Thai as well as Move Forward,” Mr Harriman told CNA’s Asia First on Monday.
However, he noted that PM Prayut has the “advantages of incumbency”.
The 250-seat Senate was not elected, but instead appointed by the previous military government, led by current PM Prayut, for a five-year term ending in mid-2024.
The body plays a key role in determining the future prime minister, as it makes the decision jointly with the House of Representatives.
“I think the people that support him will continue to support him,” said Mr Harriman, on Mr Prayut’s chances in the election after switching parties.
“I think what will impact his party and chances in the election is the fact that there will now be some vote splitting that will occur within the conservative block,” he said, adding that it could have a substantial impact on results, especially in districts where the margin was quite tight in the last contest.