SALEM -- Abruptly changing course on the legislative session's signature issue, Senate Democrats put forth a minimum wage plan Thursday that offers smaller raises to rural Oregonians while edging rates in Portland closer to $15.

The shift took the Capitol by surprise and added heat to a day that began with Republicans forcing their colleagues to read bills aloud before a series of floor votes and ended with an emotional hearing on a contentious gun-control measure.

Early Thursday, Sen. Michael Dembrow unveiled a plan for a $12.50 wage floor in so-called "frontier" counties while bumping wages to $14.75 in the Portland area. A third, middle tier that includes Eugene and Bend would get $13.75.

The Senate's unexpected proposal frustrated union leaders and threatened to derail a deal between Gov. Kate Brown and business leaders reached just last week. Brown's plan calls for two minimum wages by 2022: $14.50 in the Portland area and $13.25 everywhere else.

A powerful coalition of labor groups behind a $13.50 ballot measure immediately worked to stall Senate Democrats' plan, issuing a fiery statement accusing unspecified corporate interests of leading a "backdoor campaign of misinformation and scare tactics" to divide lawmakers. They promised to redouble efforts to collect signatures, in what amounted to an unveiled threat to legislative leaders.

"Business lobbyists continue to stir the pot and instill fear in people," Heather Conroy, executive director of Service Employees International Union Local 503, said in an interview. "Low wages are ravaging communities now."

The Senate plan could, however, shore up support among moderate rural Democrats, who have vacillated over whether to vote for an increase that they worry could hurt small businesses back home -- especially in an election year.

While most Senate Democrats did not oppose Brown's plan outright, a groundswell of support for a more rural-friendly option arose during the session's first four days. That seemed to reach its apex Monday, when Dembrow, chair of the powerful Senate Workforce committee, called off a vote scheduled on Brown's proposal, introducing his own plan instead.

"We heard a lot of concern from those frontier parts of the state they they were going to have a hard time accommodating these increases," Dembrow said. "We need to have a good night's sleep and come and have our debate."

The committee is set to discuss the new plan for the first time at 1:36 p.m. Friday, exactly 24 hours after Senate Democrats decided to run with their idea.

Senate Majority Leader Ginny Burdick, D-Portland, said the Senate plan does a better job at recognizing differences between urban and rural economies, and brushed off suggestions that the unions would prevail.

"It hits the sweet spot," Burdick said, before discussing the ballot measure: "I don't think they'll pass it. If we get a real increase, the voters will say 'thank you.'"

The plan could also soften opposition from Republicans, who have promised to fight wage-raising proposals with fury. They are already using a rare procedural measure, requiring every bill to be read, line by line, to slow down the legislative process.

But it's not clear whether it will get traction in the House. House Speaker Tina Kotek has said she wants at least $13.50. Her spokeswoman, Lindsey O'Brien, said she's still reviewing the Senate plan.

Majority Leader Jennifer Williamson, D-Portland, said she hadn't read it, either.

Williamson's main focus Thursday was helping win support for controversial gun legislation, House Bill 4147, that would ban default sales when background checks take longer than three days.

The House Judiciary Committee took up that bill, born in the wake of a mass shooting at a Charleston, S.C., church, in hours-long hearing late Thursday that saw emotional testimony from a woman whose mother and two cousins died in that attack.

The suspected gunman, federal authorities found, bought his weapon after his background check took longer than three days. If the sale had waited for the check to come back, the sale would have been denied.

"The gun dealer in this case decided to proceed with the sale even though the background check was not complete," said the Rev. Sharon Risher, whose trip to Oregon was funded by gun control group, Everytown Survivors Network. "The consequences of that decision are devastating."

Kristen Grainger, Brown's spokeswoman, indicated that Brown would remain neutral on the minimum wage discussions for now

"It's the legislative process -- a variety of discussions are ongoing," she wrote in a text message.

-- Ian K. Kullgren and Denis C. Theriault

503-294-4006; @IanKullgren