The number of veteran Liberal candidates on Kathleen Wynne's team is shrinking and that will make her task of winning re-election even tougher than it already is.

Three Liberal MPPs revealed in co-ordinated announcements on Thursday they will not run in June. This brings to 11 the number of Liberals who are throwing in the towel, nearly one-fifth of the caucus.

It's not uncommon for MPPs to quit ahead of an election. Their stated reasons for choosing not to run vary, from seeking other opportunities, to health, to retirement after long careers in office. And it's something that happens within all parties (four New Democrats won't be running again, and one Progressive Conservative).

But for so many Liberals not to seek re-election, with the party trailing in the polls, opens up room to conclude that they don't like their chances of winning. The opposition parties are only too happy to provide metaphors involving rats and a sinking ship.

The main reason the departures hurt Wynne politically is quite simple: incumbents, for the most part, tend to win.

That tendency does not hold if there is a big mood for change, as there certainly could be in this election. But campaigners from all parties will agree that it is harder for a challenger to knock off an incumbent than it is to win an open riding.

Kathleen Wynne will head into the Ontario election campaign with no more than 46 Liberal incumbents running. (CBC)

The Liberals need all the help they can get. The polls already suggest Wynne has an uphill battle to win. The incumbents quitting won't make it any easier.

These latest withdrawals means the Liberals will have at most 46 incumbents running. Ontario's expanded number of ridings means this time, to win a minority, a party needs to take at least 50 seats. The threshold for a majority is 63.

So for the Liberals to win, a decent number of their first-time candidates will need to pull off victories.

Liberal MPPs who won't seek re-election Tracy MacCharles​

Michael Chan

Grant Crack

Eric Hoskins

Brad Duguid

Deb Matthews

Liz Sandals

Dave Levac

Monte Kwinter

Mario Sergio

Glen Murray

The departures of the 11 MPPs put a bunch of ridings into play. The Liberals won by margins of less than 10 per cent in the last election in the ridings of London North Centre, Brantford-Brant and Humber River-Black Creek, according to CBC research that transposes the 2014 results to the new riding boundaries.

Incumbents not running also mean that ridings won comfortably by the Liberals in successive elections (Guelph, Scarborough Centre, York Centre, Glengarry-Prescott-Russell) become clear targets for the opposition parties.

Research by CBC polling analyst Éric Grenier suggests a party typically loses about three to five percentage points in a riding when its incumbent doesn't run. It's not huge, but it can make all the difference in a close race.