One of the D.C. Council’s longest-serving members is facing the toughest challenge of his political career after a leading candidate dropped out of the Democratic primary Wednesday, setting Jim Graham up for a one-on-one contest against a well-financed opponent.

Graham (D-Ward 1) has been viewed as vulnerable to a electoral challenge for the first time since he was first elected in 1998 after being reprimanded for an ethical lapse last year. But his foes have feared a split vote could allow him to win a fifth four-year term, leading to calls from some quarters to clear the field.

Bryan Weaver, an Adams Morgan community activist who lost to Graham in the 2010 Democratic primary and later ran unsuccessfully for an at-large seat, said Wednesday he will change his party registration to independent and contest the Ward 1 seat in the general election.

His withdrawal from the primary leaves Brianne Nadeau, a U Street public relations consultant, as the sole Democratic candidate opposing Graham. In a statement, Weaver acknowledged that continuing to campaign as a Democrat could split the anti-Graham vote.

“[U]nder the District’s current primary system, multiple candidates challenge incumbents and end up splitting the anti-incumbent vote and residents are left with ethically challenged leaders,” he said.

Running in the November election as a “progressive independent,” Weaver said, would give voters “a real choice.”

Nadeau had outraised Weaver, with more than $66,000 to Weaver’s $37,725 as of a Dec. 10 fundraising deadline. She received a major boost Tuesday when council member David P. Grosso (I-At Large) endorsed her in an automated phone call, a rare breach of legislative comity that illustrated Graham’s vulnerability.

“There’s now a clear choice in this election,” Nadeau said Wednesday. “We hope voters will show up on April 1, and they’ll be ready for change as they keep telling me they will be.”

Graham was reprimanded by his colleagues in February after an investigation found that he had improperly intermingled his responsibilities as a council member and Metro board member. The D.C. Board of Ethics and Government Accountability found “substantial” evidence that Graham suggested a businessman withdraw from a real-estate project on Metro land in exchange for his support in securing a city contract.

Graham has denied any wrongdoing, but has also offered regrets for not handling the situation differently. “If such a thing happened today, there’s no doubt in my mind what I would do,” he said in a December radio interview.

In the same interview, he sought to play down his political opposition, citing results of an internal poll showing his support at 37 percent, with his opponents in the single digits.

On Wednesday, Graham said he “welcomed the opportunity to discuss comparative achievements and records with Brianne,” citing his 15 years of public service across Ward 1, the city’s most racially and economically diverse. “I’ve worked very hard not to underestimate anyone,” he added. “We’re going to work hard.”

Nadeau on Wednesday called on Weaver backers to support her in the primary. But Weaver, in an interview, said he plans to run regardless of who emerges and declined to suggest that his Democratic supporters vote for Nadeau.

A fourth Democratic candidate who submitted ballot petitions, Beverley R. Wheeler, withdrew from the race earlier in the month, endorsing Nadeau.