ST. PAUL — A Democratic state senator will buck her party’s historical alliance with the Minnesota teachers union and push to drop seniority as the main determinant in teacher layoffs.

Sen. Terri Bonoff will formally introduce a bill aimed at the “last in, first out” layoff system common across Minnesota on Thursday. The Minnetonka lawmaker’s bill would have schools consider subject area and effectiveness in deciding layoffs, in addition to any other locally agreed criteria.

Bonoff’s bill is similar to one House Republicans introduced last week. Two Republicans were listed as co-sponsors of Bonoff’s bill, and its success in the Democrat-controlled Senate will rest largely on how many in her party she can win over.

This is not the first time a fight over teacher layoff policies has reached the Capitol. Democratic Gov. Mark Dayton vetoed a Republican push to ax the seniority-based system in 2012 and recently lashed out at a conservative group calling for its removal.

In a letter explaining his 2012 veto, Dayton called that bill — which also would have made subject area and effectiveness main layoff criteria — untested and vague.

Bonoff said Wednesday she respects teachers but believes performance, not longevity, should determine teacher layoffs.

“I think that’s what serves our students best,” she said.