The statement was issued by Senate Majority Leader Harry Reid, House Speaker Nancy Pelosi, West Virginia Gov. Joe Manchin, head of the Democratic Governors Association, and Democratic National Committee Chairman Howard Dean.
Officials said Sen. Ben Cardin of Maryland was ready to endorse Obama on Wednesday.
On Wednesday, both Obama and Clinton were addressing the American Israel Public Affairs Committee, a pro-Israel lobbying group.
On the final night of the primary season, Clinton won South Dakota while Obama took Montana - and a slew of party superdelegates who declared their support to help him clinch the party nod. He did it, according to The Associated Press tally, based on primary elections, state Democratic caucuses and support from superdelegates. It took 2,118 delegates to clinch the nomination at the convention in Denver this summer, and Obama had 2,144 by the AP count.
In contrast to the 17-month Democratic primary, Republicans gave McCain the status of likely GOP nominee in March. Since then, McCain has laid the groundwork for the general election campaign by portraying Obama as lacking the experience and judgment needed to be commander in chief.
McCain spoke first and he accused his younger rival of voting "to deny funds to the soldiers who have done a brilliant and brave job" in Iraq. It was a reference to 2007 legislation to pay for the Iraq war, a measure Obama opposed, citing the lack of a timetable for withdrawing troops.
The Republican was taking his message - that he has a record of reform while his opponent simply has rhetoric - directly to the voters in morning appearances on network news programs from Louisiana, where he will campaign later Wednesday.
Obama addressed thousands of cheering backers in the same Minnesota arena where Republicans will hold their nominating convention in early September. He promised an aboveboard debate and seemed to suggest that the GOP simply engages in divisive politics.
Said Obama: "What you don't deserve is another election that's governed by fear, and innuendo, and division. What you won't hear from this campaign or this party is the kind of politics that uses religion as a wedge and patriotism as a bludgeon - that sees our opponents not as competitors to challenge, but enemies to demonize."