Illinois state Sen. Dave Koehler, who abruptly exited the Democratic primary in the 17th Congressional District race two weeks ago, shed some light Thursday on his surprising decision.

He said he dropped out the same day U.S. Sen. Dick Durbin, D-Ill., told him he was going to endorse rival Cheri Bustos.

Koehler’s exit came on a Friday, just three days before the filing deadline for state legislative races. Over that weekend, he and supporters had to scramble to get enough signatures to get on the ballot to run for re-election to his state Senate seat.

At the time, Koehler said he decided he could best serve Illinoisans by staying in Springfield rather going to Washington, D.C., but almost immediately rumors surfaced that Durbin intervened.

The Capitol Fax blog reported shortly afterward that Durbin called Koehler to say he was going to endorse Bustos. And on Thursday, Koehler told the State Journal-Register in Springfield the same thing.

In an interview with the Quad-City Times, Koehler said he didn’t think the endorsement would have hurt him in his hometown of Peoria but it would in other parts of the district where he isn’t as well known.

“Having Senator Durbin endorse my opponent and having all that work against me was more than I could overcome,” he said. He called it a “major” factor in his decision to drop out.

Koehler said that he knows Durbin is close to Bustos’ family. Her father, Gene Callahan, was a top aide to former Sen. Alan Dixon and also worked for the late Sen. Paul Simon.

The scramble for endorsements is an important part of any primary, and Koehler had won the support of two influential unions himself.

The United Auto Workers and AFL-CIO both endorsed him, something that angered some union supporters of the other candidates, according to local Democrats. Bustos and the other candidates pointed out they hadn’t been interviewed before the endorsements were made.

Durbin hasn’t responded to an interview request on the subject, but he told a Peoria television station last week that he told Koehler that Bustos held a fundraising edge and whoever lost the primary would be out of public service. Durbin said he told Koehler, “I hope that you’ll consider staying with the state Senate.”

Bustos raised more money than Koehler in the third quarter. She’s also received the endorsement of EMILY’s List, an influential group that endorses pro-choice Democratic women.

Koehler said he wasn’t having any doubts about whether to run for the seat before hearing from Durbin, but some observers have said his legislative record may have been a target for Republicans in the general election.

In a statement Thursday, Bustos said: “We respect Sen. Koehler’s decision and appreciate his passion to keep fighting for critical priorities in the state Senate. There’s a lot at stake — and we look forward to a strong, united Democratic effort to fight for what’s right.”

Along with Bustos, Rock Island attorney Eric Reyes, Augustana official Greg Aguilar and Freeport Mayor George Gaulrapp are in the Democratic primary. The filing period for the March 20 primary is Dec. 23 through 27.