DFL Gov. Mark Dayton threatened his first veto of the year on Monday, vowing to take down a nearly $1 billion capital investment package from the DFL-led House and Senate unless legislative leaders excise a tiny provision that would ban the state from requiring sprinklers in larger new homes.

Dayton's surprise demand sparked a showdown with lawmakers in an otherwise quiet and workmanlike final week of the legislative session.

"I will not have something rammed down my throat," Dayton said at a news conference Monday.

Dayton invited reporters to his office to offer his guidance on a range of still unsettled issues, pressing for tougher payday lending restrictions, better campaign spending disclosure and a limited measure on medical marijuana.

Democratic leaders in the Senate, where the sprinkler ban originated, reacted cooly to Dayton trying to sway the legislative branch.

"I was a little surprised that he would make that strong a statement without expressing it to me first," said Senate Majority Leader Tom Bakk, DFL-Cook. "I'm not too interested in negotiating a bonding bill through the press."

Gov. Mark Dayton made a point as he spoke to the media Monday morning about his priorities for the end of the legislative session.

The blowup is over a provision that Sen. Dave Senjem, R-Rochester, slipped into the $846 million bonding bill used to borrow for new construction projects across the state — an already politically complex measure that sets aside money for buildings, roads and bridges.

Dayton lacks the authority to surgically veto policy provisions such as the sprinkler ban, so his only option is to veto the entire package if his objections are that strong.

"I think that is totally ridiculous. He would not do that," said Senjem, the ranking minority member on the capital investment committee and a former majority leader who has jousted with Dayton before. "We all know that. He is simply trying to, if you will, intimidate members of his own party."

An easy solution for Democrats would be to simply remove the measure before the final vote, but a significant wrinkle has emerged. The provision has at least some bipartisan support and Republicans now say the sprinkler ban could be critical to winning their support for the borrowing measure. State borrowing requires a supermajority of votes, giving House and Senate Republicans a rare lever of power in the closing days of the session.

Senate Minority Leader David Hann, R-Eden Prairie, said it would be foolish for Dayton to torpedo a painstakingly negotiated borrowing measure over the sprinkler issue. If the ban is not enacted, within about five months all new home construction greater than about 4,500 square feet would be required to include a system of automatic sprinklers.

"We don't have much time left and we have broad agreement on this bill as it is," Hann said.

Dayton has vetoed efforts to block the state sprinkler mandates twice before, when the Legislature was controlled by Republicans. He said he was persuaded by fire officials' arguments that home sprinklers would reduce deaths and better protect firefighters who must enter burning homes.

Fire officials have said that newer homes burn faster and that they are seeing more firefighters injured when floors collapse in new homes.

"I simply do not see how we can further jeopardize the lives of the individuals whose mission is to protect the public and who risk their lives on a daily basis," Dayton wrote in a 2011 veto letter.

Builders say that mandating sprinklers in new homes could add thousands of dollars to construction costs, putting new homes out of the reach of some buyers. Some housing experts say the sprinklers would add an average of $4,200 to the price of a new home, but others say it could be much higher.

"This is really a case of Governor Dayton ramming a $10,000-20,000 expense down the throats of new home buyers," said Shawn Nelson, president of the Builders Association of the Twin Cities. "This is simply bad policy, everyone knows that, and it's time for the governor to work with the industry and come up with a better way to do this."

Minnesota is embroiled in similar fight to one that has been raging in other states since 2009, when the sprinkler recommendation was included in the 2009 International Residential Code. California and Maryland now require sprinklers in most new homes. Pennsylvania officials imposed a residential sprinkler mandate in 2011, but Republican Gov. Tom Corbett and state legislators quickly repealed the measure.

Minnesota legislators who oppose a sprinkler mandate say the decision belongs to the homeowner.

"The homeowners really ought to be the ones making that calculation, not the Legislature," Hann said.

Unfinished business

Dayton also pressed legislators to reach agreement on a limited medical marijuana proposal.

The Senate and House have passed medical marijuana bills with little time to resolve significant differences. The House proposal would have patients consent to state-led research, puts tight limits on growth and distribution of marijuana and includes a smaller list of medical conditions that would be covered. The Senate plan has a longer list of conditions, a wider distribution network and it allows patients to possess marijuana plants that they can inhale using a vaporizer.

Dayton has supported the more limited House proposal, but suggested Monday that he's open to some provisions from the Senate bill. House and Senate sponsors are talking through their differences and a final agreement could be reached Tuesday.

Dayton also wants legislators to approve new restrictions on payday lenders. The House and Senate have vastly different measures to curtail payday lenders from predatory behavior.

He asked Senate leaders to simply vote on the tougher House measure to see if legislators "are willing to stand up to protect the interests of the people of Minnesota, rather than the economic interests of the payday lenders. And that's what it really comes down to."

Legislators expressed little interest in taking the governor's advice.

"I am having trouble finding the necessary votes to pass the bill," said Sen. Jeff Hayden, DFL-Minneapolis.

Dayton also called on the House to vote on a much-debated measure to require political groups to disclose their spending and donors. The state's largest anti-abortion organization and the National Rifle Association oppose the measure, which would force political nonprofits to reveal information that is now secret.

House Speaker Paul Thissen appeared unwilling to bow to Dayton's pressure.

"I think I have a good sense of where people stand," said Thissen, DFL-Minneapolis. "I would love that bill to pass, but I don't believe the votes are here on the House floor to do it."

Star Tribune staff writers Rachel E. Stassen-Berger, Patrick Condon and Abby Simons contributed to this report.

Baird Helgeson • 651-925-5044