BOSTON — Senator Kerry is facing his first primary opponent since he first took office 23 years ago, and his challenger has one issue in mind: The senator’s 2003 vote authorizing President Bush to launch military action against Iraq.

Ed O’Reilly said he was so incensed by Mr. Kerry’s vote that he gave up his law practice to devote himself full-time to ousting the Democratic 2004 presidential nominee from office.

“I looked at that and said ‘What kind of person would vote for a war knowing it’s wrong?'” Mr. O’Reilly, who supported Mr. Kerry’s 2004 presidential bid after initially backing Howard Dean, said. “John Kerry knew the vote was wrong but to increase his chances to become president he voted for it anyway. It was a calculated vote based on his own political ambition.”

Mr. Kerry has defended his vote, saying it was intended to give Mr. Bush a strong hand in seeking international sanctions — an option he says the White House did not pursue effectively.

Although few give Mr. O’Reilly anything more than long-shot odds at defeating Mr. Kerry, the 55-year-old lawyer and former firefighter from the fishing village of Gloucester has already scored one victory.

Earlier this month, Mr. O’Reilly received the backing of about 23% of delegates at the annual state Democratic convention — despite an effort by Mr. Kerry supporters to deny him the 15% needed to get a spot on the ballot.

“John Kerry and his campaign worked so hard from the top down in order to squelch my candidacy and I still came out with nearly one out of four of the party insiders,” he said. “That says a lot about how people feel about John Kerry.”

A recent poll found 51% of people who have voted in previous general elections said it’s time to give someone other than Mr. Kerry a chance, while 38% said the senator deserves to be re-elected. Nine percent were undecided, and 2% didn’t respond. The Suffolk University/WHDH-TV poll of 500 Massachusetts voters was conducted during June 8-10 and had a margin of error of plus or minus 4.4 percentage points.

Mr. Kerry supporters note the poll didn’t mention Mr. O’Reilly. They also say the 23% of delegates Mr. O’Reilly won at the Democratic convention included pro-Senator Clinton activists upset with Mr. Kerry’s support of Senator Obama.

A Kerry campaign manager, Roger Lau, pointed to a series of Kerry accomplishments, including legislation helping families avoid foreclosure, money for veterans’ counseling services, and $13 million in disaster assistance for the Massachusetts fishing industry.

“John Kerry is running for re-election because he wants to keep fighting to end the war in Iraq, make America energy independent, and finally achieve health care for all Americans,” Mr. Lau said.