Boston ? Before there was a Scott Brown, amazing the political world by capturing Ted Kennedy’s old Senate seat, there was a Charles D. Baker Jr., challenging the Democratic grip on Beacon Hill by announcing that he would try to deny Barack Obama’s favorite governor, Deval Patrick, a second term.

Baker, a 53-year-old Harvard grad, was no run-of-the-mill candidate. His GOP credentials were established during the years that he worked as the budget chief for former Republican Govs. William Weld and Paul Cellucci, and his business background was augmented by his more recent service as CEO of Harvard Pilgrim Health Care, the second largest insurer in the commonwealth.

But on the day he entered the race last July, Baker said he supported abortion rights and same-sex marriage. “My brother’s gay, and he’s married, and he lives in Massachusetts, so I’m for it. Is that straight enough?” he told The Boston Globe.

In case the message was missed, Baker later chose as his running mate for lieutenant governor a veteran state senator, Richard Tisei, who is openly gay.

Even more surprising, perhaps, was the back story on the Patrick-Baker relationship. Four years ago, when Patrick won over establishment Democratic opponents on his way to becoming Massachusetts’ first black governor, he sounded out Baker about becoming his running mate on what would have been a cross-party ticket. Baker said no, but later agreed to serve on Patrick’s budgetary transition team.

All this happened before Brown won the special Senate election after Kennedy’s death, raising the stakes in the gubernatorial election and catapulting Baker into prominence.

In an interview last week, Baker said he sensed the potential for the perpetually outnumbered Massachusetts Republicans, who hold only five of 40 state Senate seats, to forge a “new coalition of reformers,” drawing support from independents and disaffected Democrats.

But the path ahead for him is filled with obstacles. In the Republican primary, he faces businessman Christy Mihos, who ran for governor four years ago as an independent, and in the fall he may have to share votes with Timothy Cahill, twice elected as state treasurer on the Democratic ticket but now running against Patrick as an anti-tax independent.

Baker outraised all the other candidates in the first reporting period of the year, drawing on his support in the insurance community, and has signaled that he will fight to control the tax issue — vowing to try to roll back a 1.25 percent sales tax increase enacted by Patrick and the Democratic Legislature last year in an effort to soften the impact of the recession on schools and social services.

In the interview, Baker said his “disappointment” with Patrick began when the governor signed deficit budgets in his first two years, even before the recession hit. Now things look much grimmer.

On tax-collection day this year, Boston newspapers reported that the lower house of the Legislature had passed a budget cutting state aid to localities and schools by 4 percent, while approving for the first time legislation creating two gambling casinos and adding racetrack slot machines, in hopes they will produce millions in new revenue.

That same day, Sarah Palin was whipping up the anti-tax crowds at a tea party rally on Boston Common. Baker did not attend but as he campaigned across the state, he reminded his audiences that he had quoted George H.W. Bush’s fateful pledge, “Read my lips. No new taxes.”

In an earlier interview, Patrick told me he believed Massachusetts voters would reward him for taking a “balanced” approach to the fiscal crisis his state, like most others, has been facing — trying to minimize both tax hikes and spending cuts. Meantime, he has begun criticizing Baker for his ties to the health insurance industry, whose premium increases are causing problems for Massachusetts’ pioneering effort at universal health coverage.

A Baker victory would signal that Brown’s upset win was not a fluke. But he rightly said, “I’m still an underdog.”

— David Broder is a columnist for Washington Post Writers Group. davidbroder@washpost.com