Their speeches to Aipac focused on similar topics - with Mr Obama setting out what he would do as US president, while Mrs Clinton referred to what "the next president" should do.
She told Aipac the Democratic party's strong commitment to Israel would continue under the next Democratic president.
"I know Senator Obama understands what is at stake here," she said.
"It is an honour to call him my friend - and let me be clear, I know that Senator Obama will be a good friend to Israel."
Whoever gets the Democratic nomination to run for president will face Republican John McCain.
Mr Obama told Aipac real security came from lasting peace in the Middle East - and he would work from the start of his administration to achieve a Palestinian state alongside an Israeli one, but with Jerusalem as the undivided capital of Israel - a comment rejected by Palestinian leader Mahmoud Abbas.
Winner: Needs 2,118 delegates
Super-delegates: Obama, 389; Clinton, 282
Total delegates: Obama, 2,154; Clinton, 1,919
South Dakota and Montana (early results): Obama, 15; Clinton, 13
Turning his attention to Iran, Mr Obama said the US-led war in Iraq had emboldened the Islamic state, which posed a real, grave danger.
"My goal will be to eliminate this threat," he said.
Mr Obama said "aggressive, principled diplomacy" was needed to deal with Tehran but added that he would "always keep the threat of military action on the table to defend our security and our ally Israel".
Mr McCain's senior foreign policy adviser Randy Scheunemann criticised Mr Obama's speech, saying he presented "a false choice... that the only diplomacy that can work is with Iranian leaders".
"And Senator McCain has a very different view, that the kind of diplomacy we should pursue is with our allies," he told the AFP news agency.
He said the Israel-American relationship was a cross-party issue - but he criticised the current administration's policies in the region.
Mr Obama has paid tribute to Mrs Clinton and hinted that she would play a role in any future Obama administration.
Mrs Clinton has said she would be "open" to the idea of being Mr Obama's vice-presidential running-mate.
The final primaries of the season were held on Tuesday - with Mr Obama winning Montana and Mrs Clinton winning South Dakota.
A candidate needs 2,118 delegates to secure the nomination and Mr Obama now has the support of 2,154 delegates. Mrs Clinton has 1,919.
Correspondents say the Democratic campaign to choose a nominee has been deeply divisive which is why senior Democrats have called for the party to unite and focus on the general election.
They urged the remaining 150 or so super-delegates - party officials with a free choice over who to support at the party's selection convention in August - to make their choice by Friday.
If confirmed, Mr Obama would be the first black candidate to represent a major party in a US presidential poll.
But even before the nomination is officially decided, the Republican-led US administration has offered its congratulations to Mr Obama.
President George W Bush said it was a "historic achievement".
Secretary of State Condoleezza Rice, the first black woman to hold the post, said it showed "an extraordinary expression of the fact that 'We the people,' is beginning to mean all of us".
The BBC's Justin Webb says Mr Obama's mixed race heritage could be why he has made it this far - he runs half as a black man and half as a post-racial politician.
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