WASHINGTON — The calls started coming about three weeks ago. There were letters. There were faxes and e-mail messages. And there were phone calls, lots of phone calls, some nice and some pretty nasty.

In the three weeks leading up to her first vote on abortion, U.S. Sen. Susan Collins received at least 6,000 messages from voters concerned about her position on late-term abortions, according to her staff. Many of the calls were part of a nationally orchestrated effort to pressure senators into approving a ban on the controversial abortion procedure.

But Collins joined Sen. Olympia Snowe in voting to allow what critics call partial-birth abortions. The votes of Maine’s Republican senators played a key role as the vote fell just three votes short of a vetoproof majority of 67. Just two other Republicans voted with Snowe and Collins.

Collins said her vote was based on opinions from physicians’ groups, including the Maine Medical Association, that said the procedure was sometimes needed to protect a woman’s health. “The decision in those circumstances must rest with the physician, the woman and her family, and not with the federal government,” she said in a statement, which acknowledged the “extreme difficulty” of the vote.

Snowe agreed. “Every woman should have the right to decide for herself,” she said in a statement.

Anti-abortion activists targeted Collins and Snowe with television ads and radio spots, urging voters to pressure the senators, who support abortion rights.

Maine’s activists pointed to Monday’s decision by the American Medical Association to support banning the abortion procedure. “Collins and Snowe showed their complete and utter disdain … by today’s vote,” said Christopher Coughlan, executive director of Maine Right to Life.

The ban would prevent the procedure, in which a fetus is drawn down the birth canal feet first, and its skull is crushed before being born. The ban on partial-birth abortions would allow the procedure only to save the life of the mother — if there was no other procedure available.

Snowe and Collins would have supported the bill had it allowed the procedure to save the life or “protect the health” of the woman. Last week, Snowe co-wrote a bill with Senate Democratic leader Tom Daschle that they viewed as a compromise. It would have outlawed all abortions, not just partial births, in the final trimester.

But anti-abortion activists said that states already were allowed by Roe vs. Wade to restrict abortion in the third trimester. “The [partial-birth] procedure itself should be banned,” said Mike Heath, executive director of the Christian Civic League in Augusta.

The Daschle-Snowe bill was roundly defeated 36-64, as Collins was the only other Republican to support the proposal.