NEW YORK (AP)  -- When Mayor Michael Bloomberg introduced Barack Obama before a campaign speech Thursday, he made it clear that none of the presidential candidates has yet to win his endorsement _ a prize they've all been seeking.

After Bloomberg declared a month ago that he isn't running but might back another candidate with his wealth and support, Obama has mounted what appears to be the most aggressive effort to woo the billionaire.

Aides to the mayor and senator said he was the first to call after Bloomberg's announcement, and Obama phoned again this week to ask the mayor to give an introduction before his speech on the economy in Manhattan, and to review the text of the speech.

Democrat Hillary Rodham Clinton and Republican John McCain have both called Bloomberg since he dangled the possibility of an endorsement, but have not met with the Democrat-turned-Republican-turned-independent. Aides to both Clinton and Bloomberg say the senator and mayor are trying to work out a time to get together soon.

After Bloomberg's introduction Thursday, the Illinois Democratic senator heaped praise on the mayor, applauding his "extraordinary leadership.''

``At a time when Washington is divided in old ideological battles, he shows us what can be achieved when we bring people together to seek pragmatic solutions,'' Obama said.

``Mr. Mayor,'' he added a moment later, ``I share your determination to bring this country together to finally make progress for the American people.''

During his address, Obama highlighted the importance of investor confidence in the market _ a priority for Bloomberg _ as well as tighter regulations on Wall Street, something that Bloomberg has argued against.

Obama said the next president should expand oversight to any institution that borrows from the government; toughen capital requirements for complex financial instruments like mortgage securities; and crack down on trading activity that ``crosses the line'' to market manipulation.

Bloomberg, who worked on Wall Street before he founded his multibillion dollar financial information company, has said America's regulatory atmosphere is already so burdensome that it scares companies into taking business overseas.

When he introduced Obama, Bloomberg predicted that not everyone ``will agree with every idea, myself included.''

``But it is critical that we know exactly where each candidate stands as we make perhaps the most important decision of our lives next November,'' he said.

Later Thursday, Obama found time during a radio interview to praise Bloomberg's proposal to levy a toll on drivers in parts of Manhattan as a way to cut traffic and pollution. If the proposal isn't approved by the state Legislature before April 7, the city stands to lose hundreds of millions of dollars in federal funding that would help get it going.

Bloomberg has had less interaction with Clinton and McCain in the last month, but he has known them longer.

McCain, the presumptive Republican nominee, endorsed Bloomberg in 2001 when the billionaire was running for mayor as a businessman with no political experience _ a long shot campaign when it began. They are also social friends, and Bloomberg hosted a fundraiser at his Manhattan home for McCain in 2003.

McCain called Bloomberg to chat earlier this month, about a week after Bloomberg announced he would not run.

Clinton was the last to call, and aides say they are trying to set up a meeting.

A Clinton endorsement from Bloomberg might be a long shot. While they have worked well together as mayor and New York senator, there has been lingering iciness between them ever since 2005, when he was running for re-election _ still a Republican _ and asked her to endorse him. She backed his Democratic opponent instead, who was not expected to win.

But Clinton may have an in with Bloomberg through her husband. The former president has become friendly with the mayor in recent years and has worked with him on climate change.