WASHINGTON – Democrat Katie McGinty, the former chief of staff for Gov. Wolf, announced her campaign for Senate Tuesday, entering a Pennsylvania Democratic primary against former Congressman Joe Sestak and taking aim at the Republican incumbent, Sen. Pat Toomey.

McGinty, who had been expected to enter the race since recently resigning her position with Wolf, made her announcement in a web video and press release, pledging to help middle-class Pennsylvanians and touting her blue-collar back story as the daughter of a Philadelphia police officer and restaurant hostess.

"I am running for Senate to stand up for middle class and hard-working families who deserve a shot at the American Dream," said McGinty, a Wayne resident.

Her entry changes the dynamic of the Democratic primary and, potentially, the general election next year, in which Democrats see beating Toomey as a key to re-taking control of the Senate.

McGinty did not mention Sestak in her release announcing her candidacy, instead taking aim at Toomey.

"Sen. Toomey has become part of a Washington mess that has middle class families left out and left behind," she said. "With a record like that, it is no wonder why outside special interests like the Koch brothers have already spent millions on propping up Senator Toomey's campaign."

She promised to be a senator who pushes for "good schools, decent wages, and affordable health care."

Democratic insiders have been desperately searching for an alternative to Sestak – they first tried to push Montgomery County Commissioner Josh Shapiro into the race – and now they have a candidate who received praise for her run for governor last year, though she finished fourth in a four-person Democratic primary and has never held elective office.

Perhaps tellingly, the first Republican response to McGinty's announcement focused instead on bashing Sestak.

"Pennsylvania Democrats are so dissatisfied with Congressman Joe Sestak's candidacy that they are now placing their bets on someone who finished dead last," a year ago said Andrea Bozek, a spokeswoman for Republicans' national Senate campaign committee. Pointing to McGinty's decision to leave the governor's office in the middle of a protracted state budget fight, Bozek said McGinty's choice to "pursue her own political ambitions instead of continuing to work to solve Pennsylvania's budget problems is proof positive she values her own self interest above that of Pennsylvania's interests."

Sestak embraced the idea that the political elite would prefer someone else, saying he would be "accountable to the people of Pennsylvania -- not party insiders."

"Too much is at stake for another six years of an establishment politician," Sestak, of Delaware County, said in a news release.

McGinty faces major hurdles in trying to beat Sestak, who has a massive fund-raising lead and has consistently performed well in elections, despite some political insiders' complaints about his unorthodox, go-it-alone style.

But should she win the primary, Democrats envision a potentially history-making ticket in which McGinty runs to become Pennsylvania's first woman senator at the same time as Hillary Clinton may be running to become the first woman president. Democrats hope a left-leaning electorate can help them unseat Toomey, who narrowly won his Senate seat in 2010.

It is likely no coincidence that McGinty announced her run a day after EMILY's List, a national group that backs pro-choice Democratic candidates, put Toomey "On Notice" for joining most Republicans in voting to eliminate federal funding for Planned Parenthood. EMILY's List has been encouraging McGinty to run, and Toomey is the first Senate incumbent the group has placed on its target list.

A Toomey campaign spokesman called McGinty "a capable person."

"We welcome her to the competition of ideas, and we'll let the Democrats sort out who their candidate will be next April," said the spokesman, Steve Kelly.

McGinty previously served as Secretary of the Pennsylvania Department of  Environmental Protection during the Rendell administration and chaired the Council on Environmental Quality during the Clinton administration. She worked closely with then-Vice President Al Gore and worked on his 2000 presidential campaign.

McGinty plans to speak at a conference of the United Steelworkers in Hershey tomorrow and at a Demcoratic picnic in Allegheny County Sunday.

This post has been updated with new comments from Sestak and Toomey's campaign.

You can follow Tamari on Twitter or email him at jtamari@phillynews.com.