Honolulu’s rail line may be built with steel and cement, but the public opinion upon which it rests is a much less sturdy substance, and politicians should think how that impacts their own political foundation.

The latest public research includes the Hawaii Poll results sponsored by the Honolulu Star-Advertiser, showing 57 percent of Oahu voters think construction on the city’s rail line should continue.

The poll taken by Ward Research also showed Mayor Kirk Caldwell with a 59 percent job approval rating, which is down from a 64 percent rating one year ago.

On Wednesday, as the state Legislature meets for the first time this year, some thought will be on what is happening with the rail project. From the 76 legislators’ perspective, the thought will be more about, “How is this project going to help or hurt my chances in the coming election?”

First a bit of comparison.

Seven years ago, Mufi Hannemann was Honolulu’s mayor and was running for reelection. He had done what masterful politicians such as Gov. John Waihee and Mayor Frank Fasi had been unable to do. Hanne-mann convinced the state Legislature to allow the City Council to raise taxes to build a rail line.

Honolulu voters loved him for it. Pollster Becki Ward, with Ward Research, called Hannemann’s 80 percent job approval rating in 2008 “phenomenal.”

Even colleagues, such as the late Councilwoman Barbara Marshall who didn’t like Hannemann’s rail plans, said “The mayor does a superb job in many, many, many areas.”

Back in 2008, the city was budgeting $3.7 billion for the project and the first riders would climb on the first phase in 2012.

Since then the project has ballooned to $6.57 billion and no train will be leaving any station for the entire 20-mile run before 2021.

Besides basking in a 80 percent approval rating, Hannemann was running a train project that in 2008 was supported by 63 percent of Oahu’s voters.

As mayor, Hannemann got the good news, but now, Caldwell is getting the grief as the now-overbudget, dramatically overdue public works project lumbers from one political pitfall to another.

Hannemann also learned a lesson about fickle voters because that 2008 campaign was his last general election victory, as he went on to miss two attempts at governor and one congressional district loss.

Caldwell is now in political trouble in his own reelection campaign. The latest edition of the Star-Advertiser’s Hawaii Poll shows the 62-year-old Manoa Democrat trailing in a hypothetical three-way race with Council Chairman Ernie Martin and former Lt. Gov James “Duke” Aiona. The poll shows Aiona winning with 43 percent of the vote and Caldwell getting just 38 percent. Aiona has yet to say if he actually plans to run, other than noting on his Facebook page that “people obviously disapprove of the job Mayor Caldwell is doing.”

Caldwell won his job because he backed rail against former Gov. Ben Cayetano, who vowed to end the project.

Now Caldwell has to consider how much that support will mean in the coming election.

Richard Borreca writes on politics on Sundays, Tuesdays and Fridays. Reach him at rborreca@staradvertiser.com.