WASHINGTON -- President Obama dismissed critics of his ambitious agenda as naysayers on Monday and vowed to have health care, energy and financial regulation initiatives in place by the end of the year.

"Then we have a whole other year after that," Obama told about 400 cheering top Democratic Party fundraisers.

Sounding confident and defiant before the party stalwarts, the president pointedly went after critics of his health care plan, many of whom assert that the system needs change but object to his proposal for public health insurance as an option to private insurance.

"You'll hear a bunch of muttering and yammering and they'll say, 'Well, we agree with reform, too,"' he said. "Well, OK, if you agree with reform, then step up."

The confrontational tone delighted the partisan crowd and came as more and more Democrats voice skepticism that Obama will succeed at getting a bipartisan health care bill through Congress.

Obama ran through a list of successful administration initiatives, including a $787 billion economic stimulus, a budget resolution, a fair-pay bill and a reversal on a ban on federal funding of stem cell research.

"It's not bad for six months," he said to laughs and cheers.

But, he added: "We can't be satisfied. We should be confident in our future but not complacent."

Obama warned that critics would get louder and pundits would grow impatient -- "a time when cynicism seems to reassert itself."

The event was billed as a thank you from Obama and the Democratic National Committee to members of his presidential campaign's national finance committee and members of the DNC's National Advisory Board. No money was raised.

Obama, whose campaign broke fundraising records, last held a fundraiser for the national party committee in May in Los Angeles, amassing about $3 million in donations. May was the national party's best fundraising month of the year, with more than $8 million raised. The party is not expected to report raising as much in June.

The DNC outraised the Republican National Committee in May. But the Republican Party had $21.5 million cash on hand at month's end, whereas the Democratic Party had $12 million and a $5.6 million debt.