Barack Obama is planning to declare himself the effective winner of the long-running contest with Hillary Clinton at the close of the Kentucky and Oregon primaries tomorrow.

At that point, he should have passed one of the last remaining milestones in the race, securing more than half the 3,253 elected delegates. To exploit this he is to shun the tradition of hosting a party in one of those two states after the polls close and instead hold a rally in Iowa, a decision full of symbolism. It was in Iowa on January 3 that Obama won the first of the 49 Democratic contests so far and irretrievably damaged Clinton's reputation for invincibility.

His campaign team is anxious to bring the contest to a close to reunite the party after the longest primary season since 1980 - and one of the most acrimonious. It also wants to concentrate on the fight against the Republican presidential nominee, John McCain, in November.

But Clinton is not yet ready to surrender. "There are some people who have been saying for months that this is over, and every time they say it, the voters come back and say, 'Oh no it's not, we're not ready for it to be over,'" she told supporters in Kentucky on Saturday.

She has already said she intends to fight on until at least the last two primaries, in South Dakota and Montana on June 3. In spite of her defiance, Clinton and Obama supporters are already engaged in informal discussion about preparing for her accepting that the race is over. Clinton's team will seek concessions, possibly a cabinet post or the Obama team taking over her campaign debt of $20m (£10.2m).

The Washington Post yesterday reported that top fundraisers of the two candidates have had private talks about merging the campaigns. One of them, Mark Aronchick, a Philadelphia lawyer who has raised more than $1m for Clinton's run, said: "Only if we do this right, and see this through in the right way, will there be a chance for a full, rapid and largely complete unification of the party."

According to the Obama team, he is just 17 short of securing half of the elected delegates, who will help to choose the nominee.

"After winning 32 of 49 contests, Barack is within reach of an absolute majority. We believe that the winner of the majority of elected, pledged delegates should and will be the Democratic nominee. We need to do absolutely everything we can to help put Barack over the top," read a campaign email to supporters.

Clinton's team will counter that the decision on the nominee rests with 4,050 delegates (the 3,253 elected ones plus 797 superdelegates) and Obama will not have reached a majority of more than half of those, 2,026, by tomorrow.

Obama secured another superdelegate yesterday, bringing his overall total to 1,908, compared with Clinton's 1,718.

But he is already behaving as if the race is over and focusing on campaigning in the states he will need to win the Oval Office in November. His campaign has sent teams into battleground states to recruit volunteers and register Democratic voters, as well as preparing ads for use against McCain. "Obviously, we don't want to wake up the morning after we become the nominee and not be prepared," said Obama's campaign manager, David Plouffe.

Obama is to spend three days this week in Florida, one of the largest of the swing states and one that could be vital to Democratic chances of taking the White House. He is not planning to visit Kentucky, which is also predominantly white and which Clinton is expected to win easily. Clinton's team concedes he is likely to win Oregon.

Even if Obama lost tomorrow's primaries, the proportional representation system should award him enough delegates to reach the target of more than half the elected delegates. Kentucky has 51 delegates at stake and Oregon 52.