Multiple news outlets have already reported that Rhode Island Governor Lincoln Chafee will switch his party registration from independent to Democrat tomorrow. The move represents the culmination of a long journey from the GOP to the Democratic Party that began in 2007 when he left the party of his father, John Chafee, an archetypal New England moderate Republican Senator — pro-environment, pro-choice, progressive on taxes — and member of a tribe that no longer exists.

Chafee’s positions on multiple issues are far more in sync with today’s Democratic Party, which was the primary motivator of his decision, according to his spokesman, Christian Vareika.

“This was not a rash decision,” Vareika told me this afternoon. “The governor reached this conclusion after assessing his own principles and priorities and values, and deciding they blend very well with the national Democratic Party.”

Chafee’s priorities, Vareika continued, “are the same ones the Democratic Party puts a real focus on today: affordable public education, investing in infrastructure, equal rights, marriage equality.”

Chafee certainly has strong political motives for switching parties right now. His poll numbers are down and, having been elected in 2010 as an independent, he faces the prospects of a difficult three way race next year that could include Democratic and Republican candidates. He might have a better chance if he prevails in a Dem primary first.

Pressed on Chaffee’s political motives for switching, Vareika waved them off. “Any speculation that this was politically motivated is off base,” he said. “Who knows who will be running? If you look at his policy priorities and values there’s no question they align with the Democratic Party.”

In one sense, the allegiance to Democratic priorities isn’t surprising. Chafee is a strong supporter of President Obama, who issued a statement warmly embracing his switch. Chafee campaigned for the president in 2008 and spoke at the 2012 Democratic National Convention.

But what’s remarkable is that someone with Chafee’s priorities was once a Republican. Chafee left the GOP six years ago, largely because he had decided it had squandered its mantle as a party of fiscal responsibility. Since then, however, his view of his former party has only dimmed, if Vareika’s quotes are any indication.

“If you look at what the party has focused on in recent years, it’s increasingly been social issues that excite the base but aren’t what matters to working Americans,” Vareika said. “The governor has seen the Republican Party has become much more hostile to reaching across the aisle and compromising and finding a middle ground. The governor has the feeling that at every juncture, Republicans in Congress have worked actively to thwart the president’s agenda, not for substantive policy reasons, but for political ones.”

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.