How many MPs left their caucus? Checking the "eighth" and the "seven"

An opposition MP, Scott Aitchison (Parry Sound—Muskoka), said Sept. 30 he will resign his seat after the fall sitting. Global News reported he will be the eighth MP to leave the Official Opposition caucus since the April 28, 2025 election. A reply on X said: "Seven liberals too."

Verdict: Eight is established, counting Aitchison's announced resignation; his seat is not vacant yet. On the government side, six MPs resigned in the same window, not seven. "Lose" mixes MPs who changed caucus (still MPs) with MPs who resigned (seats vacant).

What "lose an MP" can mean

• Change of affiliation (floor cross): still an MP; seat not vacant.
• Resignation (including to the Senate): leaves the House; seat vacant.
• Seat vacated by a court: a vacancy, not a caucus resignation.

Changing affiliation triggers no byelection; a vacancy does.

The six questions

Who decides?

Each MP decides whether to resign or change affiliation. For a vacancy, the Speaker's warrant goes to the Chief Electoral Officer, and the Governor in Council sets the byelection.

What?

Official Opposition caucus departures and government-caucus resignations since the 2025 election.

When?

Election: April 28, 2025. Reports: Sept. 29–30, 2026. Aitchison's seat stays filled until he resigns, which he said will be after the fall sitting (scheduled to end Dec. 11).

Where?

The House of Commons "Changes in Membership" list.

Why check?

Two numbers are circulating as if they measure the same thing.

How?

Counts from the Commons list; news only for Aitchison's announcement.

Official Opposition caucus departures

1. Chris d'Entremont: to the government caucus, Nov. 4, 2025
2. Michael Ma: same, Dec. 11, 2025
3. Matt Jeneroux: same, Feb. 18, 2026
4. Marilyn Gladu: same, April 8, 2026
5. Richard Martel: resigned (Senate appointment), July 7, 2026
6. Cathay Wagantall: resigned, Aug. 31, 2026
7. Larry Brock: resigned, Sept. 18, 2026
8. Scott Aitchison: announced Sept. 30, 2026; seat not yet vacant, so not on the Commons list

That is Global's split: four floor crosses, one Senate move, three leaving politics. All eight fall between Nov. 4, 2025 and Sept. 30, 2026, inside a year. Not counted: Damien Kurek resigned June 17, 2025 so the Opposition Leader could run in Battle River—Crowfoot; the leader won on Aug. 18, 2025.

Government-side resignations (same window)

Chrystia Freeland (Jan. 9, 2026), Bill Blair (Feb. 2), Jonathan Wilkinson (June 19), Nathaniel Erskine-Smith (July 7), Shaun Chen (Aug. 15) and Steven Guilbeault (Aug. 28): six. No government MP changed affiliation away from the government caucus.

A seventh government-side seat event: Tatiana Auguste's Terrebonne seat was vacated Feb. 13, 2026 after the election was declared null and void. She won it back April 13. That is not a resignation.

Net picture

Commons standings today: government caucus 173, Official Opposition 137 (Aitchison included), other members 27, six vacant. Five floor crosses moved sitting MPs into the government caucus: the four above and one from a smaller caucus.

Positions

• Global News: Aitchison "will be the eighth MP to leave" the Official Opposition caucus "since the 2025 election."
• Aitchison: "After 30 years of public life, it is time for a new chapter where I will explore new opportunities."
• X reply: "Seven liberals too. Why don't you ever mention that?"

Established, claimed and unknown

• EstablishedEight Official Opposition caucus departures since the 2025 election, counting Aitchison's announced resignation. Seven are on the Commons list; his seat is not vacant yet.
• EstablishedAll eight fall within a year (Nov. 4, 2025 to Sept. 30, 2026), so Global's "in a year" headline and "since the 2025 election" body both hold.
• EstablishedSix government MPs resigned their seats in the same window.
• EstablishedFloor crosses do not vacate seats.
• Claimed (social media)"Seven" comparable government departures. Six resignations are recorded; seven only if the court-voided Terrebonne election is counted, and that MP won the seat back.
• UnknownAitchison's exact resignation date; what the X reply counted; whether more MPs leave either caucus.

History of the rule

MPs may change affiliation without vacating their seats. When a seat is vacant, a writ must be issued between the 11th and 180th day after the Chief Electoral Officer receives the Speaker's warrant (Parliament of Canada Act, s. 31).

What could change it

Parliament could legislate consequences for floor crossing; 5W1H has not checked for such bills. Current vacancies follow the 11-to-180-day window.

Sources