The weak economy hung over the convention as it dominates the election.
Obama "knows better than anyone there's more hard work to do" to fix it said Castro. He said that after the deep recession, the nation is making progress "despite incredible odds and united Republican opposition."
He declared that 4.5 million jobs have been created since the president took office — though that number refers only to private sector employment gains over the past 29 months and leaves out state and local government jobs that continue to disappear each month.
Castro, the first Hispanic chosen to deliver a keynote address, was unsparing in criticizing Romney, suggesting the former Massachusetts governor might not even be the driving force on the Republican ticket this fall.
"First they called it 'trickle down, the supply side," he said of the economic proposals backed by Republicans. "Now it's Romney/Ryan. Or is it Ryan/Romney?"
"Either way, their theory has been tested. It failed. ...Mitt Romney just doesn't get it," Castro said. Romney's running mate is Wisconsin Rep. Paul Ryan.
The divide over taxes goes to the core of the campaign.
Romney and the Republicans favor extension of all of the existing Bush-era tax cuts due to expire on Dec. 31, and also want to cut tax rates 20 percent across the board.
Obama, too, wants to keep the existing tax cuts in place — except for people with earnings of $250,000 a year or more.
Democrats criticized Romney last week for failing to offer a detailed plan to fix the economy. But nowhere in their own convention's first evening of speechmaking did anyone present proposals for reining in deficits that now exceed $1 trillion annually.
Delegates in the convention hall cheered whenever Obama's image showed on the huge screen behind the speaker's podium, and roared when the late Sen. Edward M. Kennedy was shown mocking Romney in their 1994 Senate race.
"On the issue of choice, I am pro-choice, my opponent is multiple choice," the late senator said as cheers grew louder.
Romney supported abortion rights while serving as governor; he opposes them now.
Democrats unspooled insult after insult as they took their turn the week after the Republicans had their convention in Tampa, Fla.
Illinois Gov. Pat Quinn said that Republicans had omitted mention of Romney's term as Massachusetts governor at their gathering.
"We already knew this extremely conservative man takes some pretty liberal deductions. Evidently that includes writing off all four years he served as governor," Quinn declared.
Said Senate Majority Leader Harry Reid of Nevada, speaking of Romney: "Never in modern American history has a presidential candidate tried so hard to hide himself from the people he hopes to serve."
"When you look at the one tax return he has released, it's obvious why there's been only one. We learned that he pays a lower tax rate than middle-class families. We learned he chose Swiss bank accounts and Cayman Island tax shelters over American institutions."
Obama, by contrast, was lauded for helping win approval of health care legislation and for supporting abortion rights and gay marriage.
In his campaign trip to Virginia earlier in the day, Obama told an audience at Norfolk State University that the economy will get worse if Romney wins the White House this fall and that Election Day apathy was his enemy — and theirs.
Republicans are "counting on you, maybe not to vote for Romney, but they're counting on you to feel discouraged," he said. "And they figure if you don't vote, then big oil will write our energy future, and insurance companies will write our health care plans, and politicians will dictate what a woman can or can't do when it comes to her own health."
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