CHARLOTTE, N.C. — Democrats gathered here on Tuesday to begin a brisk three-day convention intended to draw sharp programmatic differences with the Republican Party and walk a line between promoting President Obama’s first-term accomplishments while recognizing the continuing economic despair that has marked the last four years.


The convention, which was gaveled to order at 5 p.m., will feature a speech by Michelle Obama, in what Mr. Obama’s aides said would be the beginning of an attempt to prompt voters to re-examine the president’s record, particularly his health care law and his work on the economy.


In contrast with the other speakers on Tuesday night – and most of the speakers this week – Ms. Obama in her remarks will not offer criticisms of Mr. Romney, instead offering what aides described as an account of her husband’s motivations in pushing through the health care bill and the auto industry bailout. Other speakers, however, will focus on what has emerged as one of the convention’s top goals: drawing sharp contrasts with Mr. Romney.


“We want to crystallize the choice between two very different visions of the economy,” said Stephanie Cutter, Mr. Obama’s deputy campaign manager.


Before the gavel was banged, the roads and sidewalks of this medium-size Southern city were choked with traffic as delegates, guests, journalists and various hangers-on sought to find their way to the site. Mr. Obama does not arrive here until Wednesday, but security appeared tighter than it was in Tampa, Fla., where Republicans gathered last week.


Democrats awoke – many after a long night of festivities – to find that downtown Charlotte (known as Uptown) had completed its transformation into a maze of high-wire fences and security barriers. There were clusters of police officers on every corner, with squads of them patrolling the streets and sidewalks on bicycle.


As Democrats prepared to convene, Mitt Romney, the Republican presidential nominee, kept out of sight on an estate in Vermont, where he was engaged in debate preparation. By contrast, Mr. Obama campaigned during parts of the Republican convention last week. Mr. Romney’s running mate, Representative Paul D. Ryan, campaigned Tuesday in Ohio and Iowa.


Mr. Obama made his final campaign event before the convention in Norfolk, Va., on Tuesday, signaling just how critical that state is to his re-election hopes.


Democrats were also preparing to adopt the party platform, including a plank endorsing same-sex marriage — a first for either party.


Beside Mrs. Obama, the choice of speakers chosen for the convention’s first night signaled some of the organizers’ demographic priorities. The keynote address – the same speech that vaulted a young Senator Obama from Illinois to national fame in 2004 — will be delivered by Julian Castro, the mayor of San Antonio who is Latino, a crucial target for the Obama campaign.


Other speakers include Lilly Ledbetter, who has become a symbol in the women’s pay equity movement; Tammy Duckworth, who lost her legs in the Iraq war and is running for Congress in Illinois; and Nancy Keenan, an abortion rights leader.


The Obama campaign has been meticulous in reviewing the speeches at the convention with, so far at least, one notable exception: the address on Wednesday night by former President Bill Clinton. In fact, an official said the campaign had not even had a chance to see Mr. Clinton’s speech.


“All I can say is that he’s going to KILL!!” David Axelrod, a senior Obama adviser, said in an e-mail.


Despite bursts of intermittent showers Tuesday, Mr. Obama’s aides said that barring safety concerns, they were proceeding with plans to have Mr. Obama deliver his acceptance speech outdoors Thursday at the Bank of America Stadium.


“Look, we plan to be there, rain or shine, unless there’s a safety issue,” Jim Messina, Mr. Obama’s campaign manager, said at a breakfast arranged by Bloomberg News. “So we’ll continue to monitor the weather, but we hope to be there, rain or shine. It’s going to be a special moment, and we’re really excited about it.”


Whether the Obama campaign can fill all those seats — no matter the weather — was another question.


“Seventy-five thousand seat stadium Thursday night,” Diane Sawyer, the ABC anchor, said to Mr. Messina at a news forum Tuesday morning, a note of incredulity in her voice. Mr. Messina did not directly address the implication.


“Look, we are incredibly excited about the opportunity,” he said. “We think we’re going to have a great convention and we think there are going to be a bunch of people at that convention.”


Mr. Obama appeared Tuesday at Norfolk State University in Virginia, where he offered what he described as a “basic recap” of the Republican convention last week, as well as a preview of some of the themes expected to be pressed here in Charlotte.


“Basically, they said, first of all, everything is bad and it’s Obama’s fault, and Governor Romney knows the secret to creating jobs and growing the economy,” Mr. Obama said. “The only problem was, he kept it secret.”


“There was a lot of talk about ‘hard truths’ and ‘bold choices,’ but they never bothered to tell you what they were,” he added. “And when my opponent had the chance to offer his secret sauce, he did not offer a single new idea. It was just retreads of the same old policies that have been sticking it to the middle class for years.”