WASHINGTON – Barack Obama will reach a significant milestone Tuesday as he marches toward the Democratic nomination for president – a majority of pledged delegates at stake in all the primaries and caucuses.

Obama will still be short of the overall number of delegates needed to clinch the nomination, unless he were to suddenly receive an avalanche of endorsements from the party and elected officials known as superdelegates. But the Illinois senator’s campaign is touting the delegate milestone as a big step in defeating his rival, Sen. Hillary Rodham Clinton of New York.

“A clear majority of elected delegates will send an unmistakable message – the people have spoken, and they are ready for change,” Obama campaign manager David Plouffe wrote in a memo to supporters Monday.

“As we near victory in one contest, the next challenge is already heating up,” Plouffe wrote. “President Bush and Senator McCain have begun coordinating their attacks on Barack Obama in an effort to extend their failed policies for a third term.”

Obama picked up the endorsement of Sen. Robert Byrd of West Virginia Monday, less than a week after Clinton overwhelmingly won the state’s primary. Byrd is the longest serving member of the U.S. Senate.

Obama has led in pledged delegates since he won the first caucuses in Iowa on Jan. 3. The two candidates stayed close through Super Tuesday, when Democrats voted in 22 states and American Samoa. Obama won 13 more delegates than Clinton that day.

Obama built an imposing lead the following two weeks, winning 11 straight contests in states such as Louisiana, Maryland, Washington and Virginia. He led by 161 pledged delegates on Feb. 19, after victories in Wisconsin and Hawaii. The lead has been relatively unchanged ever since as the candidates spent the past three months trading victories.

Obama goes into Tuesday’s contests with 1,610.5 pledged delegates won in primaries and caucuses. He needs 17 more to reach a majority of the 3,253 pledged delegates available. Clinton has 1,443.5 pledged delegates, according to the latest tally by The Associated Press.

Clinton’s campaign played down the significance of the milestone, accusing Obama of declaring victory without reaching the required number of overall delegates.

“Premature victory laps and false declarations of victory are unwarranted. Declaring ‘mission accomplished’ does not make it so,” Howard Wolfson, Clinton’s communications director, said in a memo to supporters.

Obama has a total of 1,915 delegates overall, including endorsements from superdelegates. Clinton has 1,721, according to the latest AP count.

Obama is a little more than 100 delegates short of the 2,026 needed to clinch the nomination. He cannot make up that ground in the Kentucky and Oregon primaries because of the proportional way in which Democrats award delegates. The two states have a total of 103 delegates at stake Tuesday.

Clinton’s campaign is trying to change the math by getting the delegates seated from the Michigan and Florida primaries. Clinton won both primaries, but the states had been stripped of their delegates for violating party rules by holding their primaries before Feb. 5.

The Democratic National Committee’s rules panel is scheduled to address the issue May 31. If any of the delegates are reinstated, it would increase the number needed to clinch the nomination.