Anti-war activists were conducting a three-day sit-in at his St. Paul office, even as his Republican challenger was pummeling him as wobbly on national security. For Sen. Paul D. Wellstone (D-Minn.), the Iraq war resolution before Congress presented a lose-lose proposition likely to anger voters he needs in his tight reelection bid.

But to Wellstone there was never really much of a choice.

The 58-year-old professor-turned-senator had built a political career on standing by his convictions, which included a decided preference for international cooperation and diplomacy over war. He was not about to abandon them now, he said on a recent morning, as he put the finishing touches on a speech he was about to deliver opposing the resolution that would authorize President Bush to use force against Iraq, with or without a United Nations mandate.

"Just putting it in self-interest terms, how would I have had the enthusiasm and the fight if I had actually cast a vote I didn't believe in?" he asked. "I couldn't do that."

So far Wellstone is alone among the handful of endangered incumbents in coming out against the resolution. None of them has more at stake on the Iraq vote than Wellstone, according to political observers in Washington and Minnesota. That's partly because of the closeness of his race against Republican Norm Coleman and partly because of the senator's history of marching to his own drumbeat.

But the Minnesota race tells a broader story about how Congress's planned debates and votes this week on the war resolution could affect pivotal races in the Nov. 5 battle for control of both houses. Bush appears likely to win broad authority to invade Iraq. In the process, all incumbents will have to make a choice, and challengers will be pressed to do the same.

Political strategists disagree over how large the war issue might loom in voters' minds. But the Senate is so narrowly divided (50 Democrats, 49 Republicans and one independent), and has so many tossup races that a few thousand votes in one state -- motivated by Iraq or any other issue -- could be enough to keep the Democrats in control or return the GOP to power.

Bush's focus on threats posed by Iraqi President Saddam Hussein, especially since late summer, has shifted attention from issues such as health care and corporate accountability, which tend to favor Democrats, and elevated concern over national security, a better issue for Republicans. This has contributed to a narrowing of the Minnesota race and some other critical contests. But it does not appear decisive so far, and some Republican and Democratic nominees are approaching the issue warily, acknowledging that voters may recoil at any heavy-handed politicizing of the topic.

Coleman, a former mayor of St. Paul, has been describing Wellstone for months as an extremist on national security, accusing him of voting repeatedly to cut military spending and criticizing his vote against the Persian Gulf War in 1991. Coleman has aligned himself with Bush on Iraq and endorsed the resolution to give Bush the authority to use military forces "as he determined to be necessary and appropriate" to enforce U.N. decrees in Iraq. But he has also been careful not to sound too bellicose or political when discussing Iraq.

Within hours after Wellstone's speech opposing the resolution last week, Coleman, in Washington for a fundraiser, held a news conference to challenge the decision. "This is not a question of the senator's patriotism. It is a question of his judgment," he said later in a conference call with Minnesota reporters, which some Democrats complained privately was a way of raising doubts about both. Nor was he willing to leave Wellstone unchallenged on conscience. Supporting the president on Iraq was a "matter of conscience," too, he said.

In his speech to the Senate, Wellstone said he supported renewed efforts to disarm Iraq through unfettered U.N. inspections and opposed what he called a "preemptive, go-it-alone strategy" that could result in thousands of deaths and jeopardize the broader war on terrorism. "The United States should unite the world against Saddam and not allow him to unite forces against us," Wellstone said. "Only a broad coalition of nations, united to disarm Saddam while preserving our war on terror, is likely to succeed."

In an earlier interview, Wellstone said analysts may be right in warning he could lose independent-minded suburbanites and other largely moderate swing voters by opposing Bush, who is as popular in Minnesota as he is in the rest of the country, according to recent polls.

Before Thursday, several of these analysts had described Wellstone as caught in a vise between moderates he needs for a winning margin and his hard-core supporters who might be tempted to stay home or vote for the Green Party candidate if they felt Wellstone had betrayed them on the war issue.

Polls showed that 60 percent of Minnesotans support military action against Iraq, although most would like to see it conducted under U.N. auspices, said Minneapolis pollster Bill Morris. "People tend to see it as action against terrorism," he said.

In addition to believing that voters will not fault him for standing by his beliefs, Wellstone said he thinks many Minnesotans share his doubts about whether the United States should attack Iraq.

"I know the conventional wisdom among Republicans is that this is the issue that will do him in," Wellstone said, speaking of himself. "But I think people want you to do what you think is right. I think people want to support the president, but they're very worried about doing it alone . . . I think people in Minnesota have the same concerns that I have."

Sen. Mark Dayton (D-Minn.), who has not said how he will vote on the resolution, agreed with Wellstone's assessment. "People want to support the president in a situation like this,'' he said, "but there's more ambivalence than there was at the outset. People are trying to decide if the situation is as urgent as the administration believes."

While it is too early to tell how Wellstone's decision will play back home, some observers believe he had little choice if he was not going to sacrifice his main appeal to voters, including those who admire his principles even though they disagree with his policies.

Lawrence Jacobs, a University of Minnesota political science professor, said of Wellstone: "In the final analysis, I really don't think he had an alternative. It was a gut check for him." Over the years, "he's been able to win over moderates and independents who disagree with him but respect him. That reputation is more important to him than anything else."

Sen. Paul D. Wellstone (D-Minn.), left, talks with Richard Holbrooke, former ambassador to the U.N., at Foreign Relations Committee hearing last month on Iraq.