Back in 2010, the Chicago Tribune endorsed Republican Rep. Mark Kirk for Senate, citing his “expertise and independence.” He won.

But today, as Kirk faces a very tough reelection bid, the paper is endorsing his opponent. And it's offering a very blunt and likely controversial reason for not backing Kirk again: the stroke he suffered in 2012.

The paper's endorsement of Rep. Tammy Duckworth (D-Ill.) over Kirk is remarkable for the degree to which it references Kirk's stroke and his — in its estimation — lack of a full recovery.

And it's not just a small part of the endorsement. It's what the editorial board leads with, in fact — before it gets to Duckworth's qualifications or even mentions her at all. And it describes its reservations about Kirk's fitness for the job in detail.

Here's the operative part:

We are saddened to say we did not see that energetic, policy-driven Kirk when we met with him Oct. 3 for an endorsement interview. You'll find the video of that meeting here. Additional reporting confirmed that the senator isn't as influential an advocate in Washington as he was for more than a decade.
While a stroke by no means disqualifies anyone from public office, we cannot tiptoe around the issue of Kirk's recovery and readiness. His health is a fundamental component of this race — a hotly contested matchup that could return control of the U.S. Senate to Democrats.
We aren't physicians; Kirk's doctor attests to his good cognitive health.
But we are voters. And our reluctant judgment is that, due to forces beyond his control, Kirk no longer can perform to the fullest the job of a U.S. senator. We are unable to endorse him for another six-year term.

The editorial is sure to raise eyebrows. Kirk is considered an underdog in his reelection bid — the most likely Republican senator to lose reelection — but talking about his health and fitness for the job has generally been done behind closed doors.

Kirk has struggled with his speech and still uses a wheelchair, but as the Tribune's editorial noted, his doctor has said he's mentally fit for the job.

What's more, it's generally verboten to refer to a candidate's mental fitness for the job, especially with nothing concrete to point to. When Sen. John McCain's (R-Ariz.) primary opponent earlier this year suggested he was getting too old and that it would hamper his job performance and that he might even die in office, it was a sign of desperation — a candidate who was about to lose trying anything she could.

In this case, it's a newspaper laying bare suspicions that voters and journalists might have had, but that even Kirk's opponents have been reluctant to raise.

Kirk spokeswoman Eleni Demertzis, in a statement, denounced the paper's editorial board, which also backed Kirk when he was repeatedly winning a tough House district in the Chicago suburbs.

“A low-blow and cheap shot by the Chicago Tribune that is not based on fact or reality,” Demertzis said. “The indisputable truth is that while Congresswoman Duckworth has been rated as one of the least effective members of Congress, Senator Mark Kirk has been one of the most successful.

“Illinois families want results — not rhetoric — and that's what Sen. Kirk delivers.”