BILLINGS, Mont. — The Democratic presidential nomination contest, relegated to almost a sideshow in recent days as fireworks intensified between Democrat Barack Obama and Republican John McCain, is all but certain to pass an important milestone today as voters head to the polls in Kentucky and Oregon.

By day's end, Obama expects to have locked up a majority of the pledged delegates to the party's national convention. While not assuring Obama of the nomination in August, the achievement would signal victory is near in his battle with Hillary Rodham Clinton.

To mark the moment, Obama will appear at a rally not in one of the primary states, but in Iowa — the state whose January caucus brought Obama a win that galvanized his campaign.

The setting is meant to suggest the near-inevitability of Obama's nomination while not claiming an outright triumph that would offend Clinton loyalists whose support is needed in November.

Yesterday, Obama continued to target McCain rather than Clinton. At a stop here, the Illinois senator noted the recent resignation of five McCain campaign staffers because of their lobbying activities. Obama asserted that the presumptive Republican nominee's campaign is "being run by Washington lobbyists and paid for with their money."

Tucker Bounds, a spokesman for the Arizona senator, countered that McCain "has the strictest policy barring federal lobbyists from the campaign in history" and challenged Obama to "shed light on the long list of federal lobbyists advising him on policy issues."

The Clinton campaign, for its part, signaled that Obama's fight for the Democratic nomination isn't over. Clinton will head tomorrow to Florida, a state whose delegates she is trying to have reinstated. Obama already had planned to appear there on the same day.

Clinton's communications director, Howard Wolfson, also characterized Obama's rally in Iowa as a "plan to declare himself the Democratic nominee" and said his delegate totals don't justify that stance. "Premature victory laps and false declarations of victory are unwarranted," he said.

Political strategists not affiliated with either Democratic campaign, however, said Obama is being careful not to actually claim victory tonight.

"This is a pretty delicate situation for the Obama campaign. They're obviously going to do everything they can here to make sure Senator Clinton has a soft landing at the end of this campaign," said Bill Carrick, a Los Angeles-based Democratic strategist.

Still, he said, "Clearly calling attention to having a majority of the pledged delegates suggests that, mathematically, the campaign is coming to an end."