MADISON, Wis. — Wisconsin Democratic candidate for governor Mahlon Mitchell scored a major union endorsement Thursday, while another closed the door onthe possibility she might quit the race to seek another state Senate term.

Mitchell, the head of the state firefighters union, won the backing of the Wisconsin AFL-CIO, which represents public and private sector workers in more than 1,000 local unions statewide. It's the latest in a series of union endorsements won by Mitchell, who has also benefited from large donations from unions representing firefighters, operating engineers and others.

Meanwhile, state Sen. Kathleen Vinehout, of Alma, filed paperwork Wednesday — two days before the deadline — declaring she would not seek re-election to the Senate for a fourth term.

Vinehout's lag in submitting the paperwork had fueled questions about whether she might drop out of the crowded governor's race. She did that in 2014 after injuries sustained in a car accident forced her to cut short a run for governor.

Another Democratic lawmaker running for governor, state Rep. Dana Wachs, of Eau Claire, declared a month ago that he wasn't running for re-election.

All candidates for office face a June 1 deadline to submit the required signatures to get on the Aug. 14 primary ballot. Matt Flynn, a former state Democratic Party chairman, is the only candidate for governor to have turned in his signatures.

Five other Democrats in addition to Mitchell, Vinehout, Wachs and Flynn are expected to turn in signatures to get on the ballot. They are state Superintendent Tony Evers; Madison Mayor Paul Soglin; former state Rep. Kelda Roys; Milwaukee businessman Andy Gronik; and political activist Mike McCabe.

Milwaukee Mayor Tom Barrett last month floated the possibility of jumping into the race, which would be his fourth run for governor. But he hasn't started circulating nomination papers, with the deadline just two weeks away.

The winner of the primary will advance to face Republican Gov. Scott Walker in November. Walker has already run three television ads as part of a $1.4 million ad buy, while no Democrat has gotten on the air yet.

Walker on Thursday touted the state's unemployment level dropping to an all-time low of 2.8 percent, which he said is proof his policies over the past seven years are working. Walker also argues on the campaign trail that electing a Democrat would be bad for the economy.

Democratic Senate Minority Leader Jennifer Shilling, of La Crosse, said the state's economy is a "mess," despite the low unemployment rate.

"Republican policies that reward corporations with massive tax breaks aren't helping to grow our middle class or put more money in the pockets of hard-working families," she said.