Never take anything for granted in a low-turnout primary election.

How many times have we been taught that lesson in Illinois, and there it was, coming at us again in the Republican primary for governor.

Bruce Rauner was supposed to run away with the election after spending a gazillion dollars.

Anticipating a Rauner nomination, Gov. Pat Quinn circulated to reporters an anti-Rauner attack ad he planned to begin airing late Tuesday.

Rauner did prevail, but not before state Sen. Kirk Dillard threw a scare into his campaign by making the race much closer than expected.

If only more of us had taken Cinda Klickna’s advice.

Klickna is the president of the Illinois Education Association, one of those evil “union bosses” in Rauner’s world, and she left a message on my home answering machine on the eve of the election urging me to cross over to vote in the Republican primary against Rauner — and for Dillard.

It wasn’t anything personal. The IEA, which represents teachers, made “hundreds of thousands” of such robocalls in the days before the election, asking likely Democratic voters to pull a Republican ballot instead.

I couldn’t heed her suggestion, too worried about my fellow Democrats possibly electing a felon to the Cook County Board of commissioners.

And anyway, Dillard didn’t have a chance, did he? As it turned out, Dillard did have a chance, and the felon didn’t.

I wonder how many other Chicago-area Democrats are going to be kicking themselves Wednesday for missing out on an opportunity to end Rauner’s political career before it was born.

I wonder how many other Chicago-area reporters are going to be kicking themselves for giving up on the race because the polling showed it to be out of reach.

Dillard’s late surge came mostly from Downstate voters, who abandoned Bill Brady and Dan Rutherford, partly in response to a big push from the unions and partly at the urging of former Gov. Jim Edgar, who endorsed Dillard.

Still, looking up at that 21,000-vote deficit, Dillard was forced to face the reality of it being too little, too late for the second time in four years. It wasn’t quite a replay of the 2010 primary vote won by Brady in a recount; Dillard acknowledged as much by conceding.

For Rauner, the falloff from his big lead in the polls will raise questions about his strength going into the November election. He ended up with 40 percent of the vote, meaning 60 percent of Republicans preferred someone else.

After going unchallenged for most of the early primary season, it turns out that the Rauner campaign bleeds when opponents fight back.

Quinn left no doubt about his intention to fight back immediately with a campaign commercial asking: “Who is the real Bruce Rauner?”

The ad tries to make hay of Rauner’s campaign declaration, later retracted, that he supported reducing the minimum wage.

It’s a simple ad, but it reminds voters of a sore subject Rauner still has difficulty negotiating — and squaring with the everyman image of the Carhartt jacket and $18 watch.

With Quinn looking vulnerable after five difficult years in office, Republicans will hold nothing back in trying to recapture the governor’s office.

While I expect Quinn to give as good as he gets, he won’t have as much money at his disposal to make his case.

No matter what the national Democratic Party does to prop up Quinn financially, Rauner will not be outspent. Not only does he have his own money, but many like-minded wealthy businessman have shown they like what he’s selling — and regular voters like his pledge to “shake up Springfield.”

Quinn will need to strike early and often against Rauner to survive the coming race.

The lesson here for the rest of us will be to run it out until the very end, no matter what the early polling shows, no matter what the conventional wisdom says.

Email: markbrown@suntimes.com

Twitter: @MarkBrownCST