WASHINGTON Senator Hillary Rodham Clinton formally threw her support behind Senator Barack Obama on Saturday, clearing the way for Obama to  challenge Senator John McCain, the Republican presidential hopeful, in the November general elections. Mrs. Clinton, speaking here to an audience of advisers and supporters who had been invited to attend from across the country, used the final rally of her presidential campaign to end her bid for the presidency and endorse  her Democratic rival- Obama. She pledged that she would do what it takes to help Obama win the White House. "Today as I suspend my campaign, I congratulate him on the victory he has won and the extraordinary campaign he has won. I endorse him and throw my full support behind him and I ask of you to join me in working as hard for Barack Obama as you have for me," the New York senator said in her 28-minute address. In her last rally as a presidential candidate, Mrs. Clinton expressed deep gratitude to the voters. who had cast ballots for her. She suspended her campaign, rather than officially ending it. That's a technicality that will allow her to raise money to retire her debt and to control the delegates she won. It is not an indication that she has any intention of resuming it. Mr. Obama stayed away because he understood this was her moment. Mrs. Clinton offered nothing less than a full-throated endorsement for and embrace of Mr. Obama and his candidacy. She has said many times that she would work her heart out for the nominee, and aides said she knew that now was the time to begin to show it. In a letter on her Web site, Mrs. Clinton expressed her support for Mr. Obama in this way: "Over the course of the last 16 months, I have been privileged and touched to witness the incredible dedication and sacrifice of so many people working for our campaign. Every minute you put into helping us win, every dollar you gave to keep up the fight meant more to me than I can ever possibly tell you. Mrs. Clinton continued, saying: I "extend my congratulations to Senator Obama and my support for his candidacy. This has been a long and hard-fought campaign, but as I have always said, my differences with Senator Obama are small compared to the differences we have with Senator McCain and the Republicans." If it was an end for Mrs. Clinton, it was a new beginning for Obama. Aides said he would take advantage of the nearly nationwide reach of his expansive get-out-the-vote operation that he built during the long Democratic nomination battle as well as his considerable financial resources to try to force McCain to compete in states which are usually reliably Republican. On Monday, Obama will travel to North Carolina a state that has not voted for a Democratic presidential candidate in 32 years to start a two-week tour of speeches, town hall forums, and other appearances intended to highlight differences with McCain on the economy. From there, he heads to Missouri, which last voted for a Democrat in 1996. His first campaign swing after securing the Democratic presidential nomination last week was to Virginia, which last voted Democratic in 1964.