NAPYITAW - Myanmar’s powerful ruling party chief Shwe Mann has been ousted from his post, party members said on Thursday, apparently after losing a power struggle with President Thein Sein three months before a general election.

The feud between the two old rivals became public in a dramatic way late on Wednesday evening, when security forces surrounded the vast headquarters of the ruling Union Solidarity and Development Party (USDP) in the capital, Naypyitaw.

The United States expressed concern about the apparent use of security forces to help resolve the dispute.

Shwe Mann’s ouster from the party follows rare discord within the establishment over the role of the military, which handed power to a semi-civilian government in 2011 but retains an effective veto over changes to the political system.

Shwe Mann had built ties with Nobel laureate and opposition leader Aung San Suu Kyi, who has called repeatedly for the military to withdraw from politics. He angered the military by supporting an attempt in parliament in June to amend the constitution to limit the military’s political role.

It is unclear what the changes mean for reforms in Myanmar, but the heavy-handed involvement of the security forces smacks of the junta-era approach to political disputes.

“It’s an unwanted and extreme step,” said Khin Zaw Win, director of the Tampadipa Institute, a policy advocacy group, and advisor to multiple MPs.

“They are using security apparatus to affect a reshuffle in the party. They should have settled this some other way rather than using brute force. This is a reminder of the very unhappy past.”

Shwe Mann was replaced as chairman of the party because he was busy in his role as speaker of parliament, the USDP said in a statement on Thursday that made no reference to the role of the security forces. President Thein Sein “transferred the role of chairman” to Htay Oo, the statement said. Htay Oo is an ally of the president.

The USDP named one of the president’s closest aides, Tin Naing Thein, as secretary general of the party. He replaced Shwe Mann supporter Maung Maung Thein.

A list of Central Executive Committee (CEC) members issued by the USDP on Thursday showed other allies of Shwe Mann have been replaced by politicians moved out of government positions in a presidential reshuffle just hours before the overnight drama unfolded.

Shwe Mann would remain a party member and contest the election in his own constituency, Than Tun, the head of the CEC, told reporters on Thursday.

Thein Sein spearheaded reforms after taking power in 2011 that initially surprised the world with their speed and breadth. But reforms have stalled as the election has come into focus.

Both Thein Sein and Shwe Mann have suggested they would accept the job of president after the Nov. 8 parliamentary election, the first free election in Myanmar in 25 years.

Tension rose between the two former top military officers over the party’s selection of electoral candidates, party sources said.

On Wednesday, the USDP selected only 59 of 159 senior officers who retired from the military to stand, the party sources said.

The decision likely angered officers and politicians keen to preserve the military’s sway. The USDP is comprised largely of former military officers and was created from a social movement established by the former junta.

Late on Wednesday, several trucks carrying soldiers and police officers arrived at the party’s headquarters.

After the security forces took control of the compound, Htay Oo and another of the president’s supporters, Soe Tha, led a late-night meeting of senior party officials that lasted into the early hours of Thursday, party members said.

Shwe Mann and high-ranking party members considered to belong to his faction were absent from the meeting, the sources said.

The security forces left after the meeting concluded at around 2:30 a.m., sources said.

The US embassy in Yangon said it was important that the government clarify what actions it has taken and their legal basis.

“Maintaining public trust in the democratic process will be critical throughout this election season and ultimately the transition to a new government next April that reflects the will of the people,” an embassy spokesperson said.

There was no additional security presence on Thursday at the enormous walled and fenced neo-classical building that houses the USDP, a Reuters witness said.

One USDP official said on Thursday the deployment was a “precaution” to guarantee the safety of politicians in the complex. Another said the presence of security forces was intended to intimidate Shwe Mann’s backers.

Shwe Mann has been at his home in Naypyitaw throughout, a family member told Reuters.

Shwe Mann reached a higher rank in the military than Thein Sein and was a presidential hopeful when the military handed over power after 49 years of rule in 2011.

The USDP is expected to fare poorly against Suu Kyi’s National League for Democracy in the election. The NLD boycotted the last election in 2010, leaving the USDP to win it by a landslide. Competition from the NLD this time will make safe seats a scarce commodity for the USDP.

The constitution reserves 25 percent of seats in parliament for unelected military officers. Changes to the constitution require the support of at least 75 percent of lawmakers, giving the military block an effective veto over changes.

An amendment that would have seen the threshold of support lowered to 70 percent failed, as expected, to gain enough support with lawmakers in a June vote.