ST. PAUL, Minn. — Minnesota business and labor union organizations were among the heaviest spenders on lobbying during the first five months of 2013, which coincided with the Legislature's annual session, according to financial reports made public Tuesday.

A trio of business groups combined to spend almost $2.1 million, largely to counter proposals in the Democratic-dominated Capitol. On the flip side, several organizations lobbying for labor union proposals reported five- or six-figure spending.

The main groups on both sides of the gay marriage debate saw a disparity in spending. Minnesotans United for All Families spent about $1.6 million in a successful effort, which was far more than the $211,000 spent by opposition group Minnesota for Marriage.

The National Rifle Association spent roughly $100,000 in a session that produced only minor gun law changes. Gun control advocates devoted far less money to enacting more restrictions.

Even though the session is over, the lobbying costs for the year could grow because some expenses don't have to be reported until later.

Among the notable reports:

— Xcel Energy Services Inc., Minnesota's dominant power company, spent about $1.16 million. Almost all of that was spent on lobbying utilities regulators rather than trying to sway lawmakers.

— Mayo Clinic, which won a long-term state commitment to aid in a major expansion, deployed about $121,000.

— Education Minnesota, the state teachers union, spent more than $658,000 in the year's first five months. Much of it was devoted to promoting investment in education.

— The American Federation of State County and Municipal Employees Council 5 directed $218,000 to efforts to influence lawmakers. The union's most visible legislative effort was to allow for the unionization of certain in-home day care workers, a measure that passed.

— The business lobby went all out to try to repel bills it found hostile. The Minnesota Chamber of Commerce spent $875,000 on legislative and administrative lobbying, the Minnesota Business Partnership put in $535,000 and the Coalition of Minnesota Businesses added another $672,000.

— The two groups on competing sides of a debate over nurse staffing rules for hospitals were closely matched. The Minnesota Nurses Association reported spending $101,000 and the Minnesota Hospitals Association countered with $84,000. They eventually reached a roundly backed compromise to study staffing of hospital units rather than have mandated nurse-to-patient ratios.