WASHINGTON — The results out of Kentucky and Oregon tonight are likely to be bittersweet for Senator Obama as he looks to claim a key victory in the Democratic primary battle even as a significant bloc of party voters refuses to ratify his nomination.

Polls are predicting an Obama victory in Oregon, and the Illinois senator will clinch a majority of total pledged delegates in the nomination fight, an achievement that his campaign is calling a “major milestone” reflecting the will of Democratic voters nationwide. Mr. Obama will stop short of declaring victory outright, but the moment will increase pressure on the remaining undecided superdelegates to endorse his candidacy and effectively end the last hopes of Senator Clinton.

“A clear majority of elected delegates will send an unmistakable message — the people have spoken, and they are ready for change,” Mr. Obama’s campaign manager, David Plouffe, wrote in an e-mail to supporters yesterday. The candidate plans to address supporters tonight in Iowa, the site of his momentous first victory in January and a potential general election battleground.

At the same time, Mr. Obama is facing the prospect of his second lopsided loss to Mrs. Clinton in as many weeks, with polls showing a nearly 30-point lead for the former first lady in Kentucky. The expected split offers a snapshot of a divided Democratic Party in which Mr. Obama has yet to win over white, working-class voters even as his nomination appears all but certain.

Surveys released yesterday by Suffolk University showed Mr. Obama ahead by four points in Oregon, 45% to 41%, and Mrs. Clinton holding a commanding advantage in Kentucky, 51% to 25%, with the remaining respondents either uncommitted or planning to cast a ballot for John Edwards, who dropped out of the race in January and endorsed Mr. Obama last week.

While respondents expressed similar views of Mrs. Clinton in both states, they differed widely on Mr. Obama, the director of Suffolk’s Political Research Center, David Paleologos, said. Seventy-three percent of Democratic voters in Oregon had a favorable view of Mr. Obama, while 43% of Kentucky Democrats viewed him favorably.

“This is a huge hurdle for November,” Mr. Paleologos said.

Analysts attribute the disparity between the two states almost entirely to demographics. While Kentucky shares the large blue-collar and predominantly white population of West Virginia, voters in Oregon tend to have higher incomes and education levels, and more identify as liberal — characteristics that have favored Mr. Obama throughout the primary season. “People here are responding to Obama as a change agent,” a professor of political science at Pacific University, near Portland, James Moore, said.

Oregon also votes entirely by mail, meaning half of the ballots in today’s primary have already been cast. While the Suffolk poll put Mr. Obama ahead by four points, other surveys and political analysts said they expect his margin of victory to be wider.

With contests in Montana, South Dakota, and Puerto Rico looming in the coming weeks, Mrs. Clinton has shown no desire to exit the race. She told voters in Kentucky yesterday that the primary campaign was “nowhere near over,” and she is claiming a lead in one measure of the nationwide popular vote. Her tally includes the votes cast in Michigan, where Mr. Obama was not on the ballot, and Florida, where neither candidate campaigned because of a dispute over party rules. Mr. Obama, as his campaign is quick to note, leads in most other measures of the popular vote, along with total delegates and states won.

The Clinton campaign took umbrage yesterday at a report that Mr. Obama would claim victory, issuing a memo titled “Mission Accomplished? Not so fast” — a none-too-subtle reference to President Bush’s infamous 2003 declaration of “mission accomplished” in the Iraq war.

Declaring victory “is a slap in the face to the millions of voters in the remaining primary states and to Senator Clinton’s 17 million supporters,” Mrs. Clinton’s communications director, Howard Wolfson, wrote in the memo. “There is no scenario under the rules of the Democratic National Committee by which Senator Obama will be able to claim the nomination tomorrow night.”

He added: “Premature victory laps and false declarations of victory are unwarranted. Declaring mission accomplished does not make it so.”

Mr. Obama moved closer to the nomination yesterday, announcing the endorsements of five more superdelegates, including the oldest and longest-serving member of the Senate, Robert Byrd of West Virginia. Mr. Byrd had remained neutral through his state’s primary last week, which Mrs. Clinton won by more than 40 points.