Even experienced political observers are slightly incredulous at what has happened over the past week in Massachusetts: a Republican state senator most well known for posing nude in Cosmo 28 years ago and for his daughter’s appearance on “American Idol” is within striking distance of taking Ted Kennedy’s Senate seat – and possibly health-care reform – from the Democrats.

And Scott Brown has momentum.

“As a resident and voter and student of politics, it feels extremely, extremely close,” said John Della Volpe, an independent pollster and new media strategist in Boston with many years of involvement in Massachusetts politics.

“We’ve seen something viral in the old way of viral, people talking to one another. That’s what people are talking about: can you believe Scott Brown is this close? That’s what everybody is talking about,” Della Volpe said.

The question is: How did a Republican get this close to the Senate seat occupied for 47 years by Democratic political royalty?

Washington shorthanders regarded it as evidence that the anger at government spending and the Obama health-care plan — on display in town hall meetings across the country last summer — was rearing its head even in one of the country’s most liberal states.

“The very fact that this race is even close is a sign of how unhappy Americans are with Democratic rule in Washington,” Newt Gingrich wrote on his website.

Massachusetts politicos said that while anti-Washington sentiment is an element of what is happening in their state, they also blame state political dynamics in combination with presumption by the Democrats and the party’s candidate – Attorney General Martha Coakley – that the seat would be theirs without much of an effort.

The Kennedy-anointed Coakley took nearly a week off from the campaign around Christmas.

“A lot of Democrats in Massachusetts and certainly the Coakley campaign and myself thought this was going to be a lot easier than it’s turning out to be,” said David Kravitz, a Boston lawyer and opera singer who runs a liberal political blog called bluemassgroup.com.

Paul Cellucci*, the state’s Republican governor from 1999 to 2001, said, “Democrats thought this was a safe seat, they were coasting and they did not measure the discontent among the voters.”

Celucci said the issues driving voter anger are as much about state politics as they are national.

“We’re having fiscal problems. The rainy day fund has been depleted. The governor raised the sales tax. Balancing the budget continues to be a significant issue and problem. It just appears that they can’t get things done,” Celucci said.

In addition, former House speaker Salvatore DiMasi in October became the third Democratic speaker in a row to be indicted, on charges of extorting profits from a property management company owned by friends. He faces 185 years in jail.

Brown, who in Monday night’s lone debate of the campaign proclaimed he would, if elected, cast the vote to stop the Democrats’ health-care bill, has been campaigning on a platform of fiscal responsibility.

“Scott came in and made people see he is going to be the jobs senator and focus on improving the economy and bringing jobs back to Massachusetts,” the state GOP chairman, Jennifer Nassour, said.

Della Volpe, who no longer works with either political party but has in the past worked with Massachusetts Democrats, said that “people are beginning to want something new.”

“They respect [Brown] in this pickup truck driving around. Everybody identifies him as this upstart candidate in a pickup truck who’s taking it to the establishment.”

David King, a professor of public policy at Harvard’s Kennedy School, said that the national health-care issue was a minor factor in the election.

“Health care is not the issue up here. It’s fiscal responsibility and state-oriented issues. It’s almost been lost on people that this is an election with national consequences,” he said.

Since the weekend, Brown has appeared to be the candidate with momentum, while Coakley has committed a number of missteps. She said in the debate that al-Qaida terrorists are “gone” from Afghanistan and only remain in Yemen and Pakistan, she came to Washington on Tuesday to raise money at a fundraiser hosted by lobbyists for large health insurance and drug companies, and on her way out of the event one of her political operatives pushed a reporter to the ground.

Meanwhile, Brown on Monday raised $1.3 million, and scored the soundbyte of Monday’s debate by rebuking moderator David Gergen for referring to “Teddy Kennedy’s seat.”

“With all due respect it’s not the Kennedy seat and it’s not the Democrats’ seat. It’s the people’s seat,” Brown said.

The latest poll, released Tuesday evening by Rasmussen Reports, showed Coakley up by two points, 49 percent to 47 percent, after leading by nine points a week before.

All the major organs of the Democratic political machine lunged into motion on Tuesday. The Democratic Senatorial Campaign Committee bought $600,000 in TV ads. The Democratic National Committee bought $567,000 of air time. The Service Employees International Union put out their own ad.

“When voters learn about Scott Brown’s true positions and the fact that he will be a lockstep vote with the extreme right-wing of his party, they will understand that Martha Coakley is the candidate who will stand up and fight for them,” said Coakley spokesman Corey Welford, in an e-mail.

Former President Bill Clinton and Sen. John Kerry, a Massachusetts Democrat, are headlining a rally for Coakley on Friday in Boston. The White House maintained Wednesday evening that President Obama had no plans to campaign for Coakley, though press secretary Robert Gibbs said there was “a lot at stake.”

Brown got support of his own from the outside, as the U.S. Chamber of Commerce began running ads on Tuesday and former New York Mayor Rudy Giuliani is scheduled to campaign with him this week.

*A misspelling of the governor’s last name was corrected.