* Yesterday, Larry Sabato moved the Illinois governor’s race from lean to likely Democratic. Today is Charlie Cook’s turn…

When it comes to rating races, it has long been our practice not to move extremely vulnerable incumbents into the other party’s territory until well into the election cycle – generally around Labor Day. Even then, they rarely move further than Lean. There are lots of good reasons for this policy, most of which grew out of lessons learned the hard way.

Republican Gov. Bruce Rauner has held the dubious distinction of being the most vulnerable incumbent of the cycle, and despite much heckling, has been sitting in the Toss Up column. But, the race recently hit a tipping point that moves it into the Lean Democrat column.

There are lots of reasons not to jump the gun on what amounts to waving the white flag on an incumbent’s chances for re-election. Some of them include the power of incumbency, the competence of an opponent’s campaign, and the increasing unreliability of public polls. All three factors collided in 2016 when Republican U.S. Sen. Ron Johnson of Wisconsin was seeking re-election and former Democratic U.S. Sen. Russ Feingold, whom Johnson had defeated six years earlier, was running to avenge his loss. There were 56 general election polls in that race, and Johnson was only ahead in four of them. Given those statistics, Johnson was moved into the Lean Democratic column pretty early in 2016. But, 15 of the 56 surveys went into the field between October 15 and Election Day, and Johnson was (barely) ahead in three of them. Johnson won the contest, 50 percent to 47 percent for Feingold and 3 percent for a Libertarian candidate. Of course, it wasn’t until after the election when Democratic operatives shredded Feingold’s campaign, holding it up as an example of malpractice. We put Johnson back into the Toss Up column 10 days before the election and no amount of second-guessing will resolve the question of whether Johnson should ever have left the Toss Up column.

One factor guaranteed to hurt an incumbent locked in a close race is the presence of one or more third-party candidates on the ballot. Again in 2016, GOP incumbent U.S. Sen. Kelly Ayotte of New Hampshire lost her re-election bid to Democrat Maggie Hassan by just 1,017 votes. But, there were two other candidates on the ballot running to Ayotte’s right; they combined for 30,339 votes, costing Ayotte the election. There are similar stories from statewide races in Montana in 2006 where Libertarians cost Republicans elections, and multiple elections in New Mexico in which Green Party candidates undercut Democratic nominees. It is rare when third party and independent candidates are truly competitive. More often than not they simply serve as spoilers for one party or the other.

It is the presence of a Conservative Party candidate and a Libertarian on the ballot that has created the latest obstacle to re-election for Rauner, and is the tipping point that moves the race into the Lean Democrat column. Rauner has had a very difficult tenure. First, he is a Republican in a very blue state, and while he is relatively moderate, he hasn’t enjoyed the same levels of popularity and success that fellow GOP Govs. Charlie Baker of Massachusetts or Larry Hogan of Maryland have experienced. His job ratings have sunk under the weight of a long-running battle (and its aftermath) over the budget, and adversarial relationships with state House Speaker Mike Madigan and Chicago Mayor Rahm Emanuel, who have proven adept at robbing Rauner of victories.

There’s more, including reliance on what I would consider some dubious polling, but whatevs. A pundit consensus is most definitely building.