THE PROMISES OF PRIMARY PRESSURE…. Sen. Arlen Specter of Pennsylvania, up until fairly recently, opposed including a public option in health care reform. This week, he reversed course. I wonder why that is.

Speaking [on Thursday] to a large and animated crowd of union organizers and health reform advocates in a brewing house just North of the Capitol, Sen. Arlen Specter (D-PA) said he supports a public insurance option.

“Schumer has it right about having a public component,” Specter said. […]

[T]he Schumer proposal is in line with the principles of the major reform campaign Health Care for America Now — and, as such, just about every major health care and labor organization in the country.

Before Specter switched parties this spring — and for a brief period afterward — he said he did not support the public option. But as a Democrat he’s facing different pressures — notably from Rep. Joe Sestak (D-PA) who plans to challenge Specter in next year’s primary — and he’s begun tacking to the left as a result.

He certainly has. In his first week as a Democrat, Specter voted against the Democratic budget, rejected a Democratic measure to help prevent mortgage foreclosures, announced his opposition to the Democratic president’s OLC nominee, announced his opposition to a public option, and told a national television audience that he’d never promised anyone he’d be a “loyal Democrat.” Soon after, he told the New York Times, “There’s still time for the Minnesota courts to do justice and declare Norm Coleman the winner.” (Specter later said he forgot he was a Democrat.)

And yet, now we see Specter speaking at HCAN rallies and endorsing a public option.

Is there any doubt that a likely primary challenge from Rep. Joe Sestak is having an effect on Specter’s policy positions?

When it comes to primary challenges, I tend to think they should be evaluated on a case-by-case basis. If Specter were a Republican-turned-Democrat from Alabama, and a primary challenger were pushing him far enough to the left to make a general election campaign more difficult, I can imagine this being detrimental.

But for those who believe primary campaigns are always harmful to a party’s interests, Specter’s example is pretty compelling evidence to the contrary.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.