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Post 1/3. Estimating the True Decline in Support for Pheu Thai: The Effect of Electoral Reform

Updated: Jun 7, 2019

By Joel Selway and Allen Hicken

In a series of three posts we estimate the decline of support for Pheu Thai by 1/. simulating results under the electoral rules used in 2011, 2/. estimating Pheu Thai’s vote share if it had run in all constituencies, and then 3/. estimating seat count and recalculating final seat tallies under the 2011 electoral rules. This enables us to say what proportion of its decline is related to a combination of holding elections under military rule and genuine loss of support.


Pheu Thai (PT)—including the previous incarnations of the party—has been the most successful party in Thai elections since 2001. In the 2011 elections, PT won 53% of seats on 48% of the votes. On March 24, 2019, Thais again went to the polls under a new constitution drawn up by the military junta that had ousted the PT government headed by Yingluck Shinawatra in May 2014. This new constitution was designed to re-fragment the party system and make it difficult for PT to capture a majority. In terms of those goals the reforms were wildly successful. PT still managed to win the most seats, capturing 137 of the 500, but instead of controlling a majority, it now controls only 27.4% of the lower house. This reflects a dramatic decline in its vote share, from 44.3% of the constituency tier votes in 2011 to 22.3% in 2019, giving it second place behind the junta-backed Palang Pracharat party (PPRP), which captured 23.7% of the votes.

Analyzing the reasons for PT’s decline is not a straightforward exercise. Each of the following was a contributing factor:

1) An electoral environment systematically biased in favor of the junta and PPRP

2) New electoral rules that decreased the advantage for larger parties

3) The fact that PT chose to contest in only 250 of the available 350 constituency seats

4) A genuine decline in popular support for PT and its leadership

How you evaluate the long-term prospects for PT depends very much on which factor you assign the most weight. Factor number 4 is obviously the most worrying from the party’s perspective. But if its decline in 2019 is largely due to some combination of 1, 2 and 3, then it is more likely that the party’s fortunes will rebound.

In this series of three posts, we try and isolate and estimate the effects of factors #2 and #3. How much of PT’s decline is the result of the new electoral rules, how much is a result of its decision to only contest 250 of the 350 constituency seats? Using a series of simulations we attempt to answer these questions.

Estimating the Effects of MMM/MMP

In this first post, we take a preliminary crack at estimating the effect of the change in electoral rules. We will return to electoral rules again at the end of the third post, once—in posts 2 & 3—we simulate how PT may have fared if it had run in all 350 constituencies.

As detailed in this post, the 2017 Constitution of Thailand changed the electoral system from a mixed-member majoritarian (MMM) system to a mixed member proportional (MMP) system, which the drafters dubbed a mixed-member apportionment system (MMA).[1] MMP produces more proportional outcomes, and thus is more accommodating to small parties, at the expense of larger parties. So, what was the effect the new electoral rules? To begin with, note that the party’s share of constituency seats was down from 2019, but not dramatically so. In 2019 the party captured 39.1% of the constituency seats, compared to 44.3% in 2011. The big difference between the two elections is the number of party list seats the party secured. In 2011 PT won 48.8% of the party list seats. In 2019 it failed to capture any seats from the party list. If we assume that vote totals remained the same, how would PT had faired if Thailand had retained MMM for the 2019 election?

Figure 1

In Figure 1, we compare the seat distribution using MMP with what it would had been if Thailand had retained MMM.[2] Using MMM nets PT an additional 36 seats from the party list, boosting its total seat share to 34 percent of the House. Palang Pracharat, as the largest vote getter, also benefits from MMM, picking up 15 more party list seats than it does under MMP. These gains come at the expense of more seats for Future Forward and small parties. In the final analysis it appears that the switch to MMP cost PT about 34 seats, enough to reduce its seat share by about 7 percentage points.

In the next post, we consider the effect of PT's choice not to contest 100 constituency seats.



[1]It also moved to a single, fused vote, the effects of which are explored here.

[2]We are assuming that the ECT will use something close to Scenario 2 as described here, and that extra party list seats would be distributed using largest remainder method.

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