Big checks from national gay and lesbian groups are pouring into Oregon in hopes of defeating a November ballot measure that would amend the state constitution by defining marriage as between a man and a woman.

"I think we have nothing to hide. We are asking for support to defeat an amendment that we certainly didn't bring to the ballot," said Rebekah Kassell, spokeswoman for No on 36 (search).

The National Gay and Lesbian Task Force (search) has given $500,000 to Oregon's "No on 36" effort and the Human Rights Campaign (search) is pledging an additional $100,000.

The groups are giving almost no money to defeat nearly identical ballot measures in 11 other states — an admission that the gay-marriage bans are likely to score easy victories in all but Oregon.

"It makes the most sense to invest your dollars where they can make the most impact and Oregon is that place," said HRC National Field Director Seth Kilbourn.

Oregon's gay-marriage opponents are expecting to be outspent by about $1 million. They are accusing the national gay and lesbian groups of meddling by not only sending money, but also dozens of campaign volunteers.

"I do think it's important that Oregonians understand that when there's a knock on the door, chances are the person on the other side is not from Oregon and that dollars are flowing into Oregon from around the country," said Georgene Rice, spokeswoman for the Defense of Marriage Coalition (search).

Voters in Missouri proved in August that money does not guarantee success. The gay-marriage ban passed by a huge margin, although opponents had a 40-to-1 spending advantage.

But Oregon, where hundreds of same-sex weddings were performed this year before the courts stepped in, is likely to be a close battleground right through the election.

Click on the video box at the top of this story to watch a report by FOX News' Dan Springer.