WASHINGTON – New England’s 12 senators span the political spectrum, from Massachusetts’ liberal lion to the more conservative thinkers in New Hampshire, but they are finding a common ground on one of the political season’s most contentious issues: gay marriage.

At least four of the region’s five Republicans will join the six Democrats and one independent and vote against the proposed constitutional amendment banning gay marriage this week. Only one, Sen. John Sununu, R-N.H., was still undecided, and calls to his office Monday were not returned.

Bonding the New England legislators is a sense that while most don’t approve of gay marriage, they say it does not belong in the U.S. Constitution and is an issue for the states, not the federal government, to decide.

Maine’s two Republican senators, Olympia Snowe and Susan Collins, Connecticut Democratic Sen. Joe Lieberman and Democratic Sen. Jack Reed of Rhode Island oppose gay marriage but agree it is a state issue.

“States currently possess the authority to decide whether or not to recognize an out-of-state marriage,” said Collins, adding that she supports the Defense of Marriage Act passed by Congress several years ago. As long as that law is on the books, she said, there is no need to amend the Constitution.

The Defense of Marriage Act defines marriage as the union of a man and a woman, and says that same-sex unions recognized by one state do not have to be recognized by others.

Supporters of the amendment, such as Texas Sen. John Cornyn, a Republican, have argued that the federal government must step in to preserve traditional marriage and ensure that state courts don’t rewrite laws in ways that erode families.

While it is not uncommon for the diverse group of Northeasterners to combine forces on regional issues such as fishing or highway funding, they often part company on social matters, such as abortion or gun control.

The Republicans from Maine and Rhode Island are moderates who are known to break ranks with their party. But throw in Sununu and his Granite State Republican counterpart, Sen. Judd Gregg, and the ideological expanse widens a bit.

Except on gay marriage.

“Marriage should be between a man and a woman,” said Gregg, a senator who received a 100 percent rating from the Christian Coalition last year, and a 14 percent rating from a leading gay rights group, the Human Rights Campaign. But, he adds, “a federal constitutional amendment is premature at this time, as federal law already gives authority to the states in recognizing marriage.”

Massachusetts’ two liberal Democrats – Sens. Edward M. Kennedy and John Kerry – received opposite marks: a zero from the Christian Coalition and 100 percent from the HRC. But they largely agree with Gregg on the amendment banning gay marriage.

It violates state sovereignty, said Kennedy, adding that Congress “has better things to do than write bigotry and prejudice into the Constitution.”

Overall, their views mesh with their constituents. In a Pew Research Center poll, 57 percent of Northeasterners said they opposed the constitutional amendment, compared to 53 percent nationwide and 47 percent in the South.

“New Hampshire has a history of saying, let people do what they want to, and they don’t like the federal government telling states what to do,” said Andy Smith, director of the Survey Center at the University of New Hampshire. “Live free or die. We’ll decide for ourselves.”

A survey of state residents, he said, showed that 55 percent support legalizing gay marriage and 64 percent oppose an amendment banning it.

Connecticut Sen. Christopher J. Dodd, a Democrat, supports civil unions and said marriage should continue to be regulated by the states.

Probably the most fervent Republican on the issue is Sen. Lincoln Chafee of Rhode Island, who slammed the proposed amendment as “nuts.”

In Vermont, where the law allows for civil unions, the two senators are, not surprisingly, on the same page. Sen. Patrick Leahy has led the opposition to the amendment in the Senate Judiciary Committee, where he is the ranking Democrat. And Sen. Jim Jeffords, an independent, said it is “really sad that the Republican leadership would bow to religious conservatives and use the Senate floor as a warm-up for the political conventions.”