WASHINGTON — When General David Petraeus briefs Congress tomorrow, he will be walking into the most contested Democratic presidential primary since 1968.

While Democratic congressional leaders have expressed skepticism about progress in Iraq ahead of the general’s testimony on the state of the war, the two Democrats vying for their party’s presidential nomination are expected show restraint and respect in their questioning.

The action kicks off tomorrow morning, when General Petraeus testifies before the Senate Armed Services Committee. Both Senator McCain, the presumptive Republican nominee for president, and Senator Clinton, who is battling Senator Obama for the Democratic Party’s nomination, sit on that panel. In the afternoon, General Petraeus will testify before the Senate Foreign Relations Committee, where Mr. Obama will hear his briefing.

A spokesman for the Senate office of Mrs. Clinton, Philippe Reines, told The New York Sun that the senator is “looking forward to the testimony of General Petraeus and Ambassador Crocker before the Armed Services Committee and expects that the committee will question them closely on the lack of political progress in Iraq, the rising influence of Iran in Iraq and the region, and the strain on our military caused by the continuing presence of large numbers of troops in Iraq.”

Mr. Obama told reporters on his plane on Friday: “We still don’t have a good answer to the question posed by Senator Warner the last time General Petraeus appeared. How has this effort in Iraq made us safer? And how do we expect that it will make us safer in the long term?”

The political posture of the Democrats seeking the White House will differ from that of their Republican rival for the presidency. Mr. McCain today is scheduled to deliver a speech to the Veterans of Foreign Wars on the importance of the Iraq war surge.

“Senator McCain is going to detail the dramatic gains that have been achieved by the new strategy over the past 14 months and the paucity of alternatives offered by senators Clinton and Obama,” the campaign’s national security director, Randy Scheunemann, said in an interview.

The approach of the two Democratic front-runners will also contrast with those of members of the House, where General Petraeus will testify on Wednesday. Last week, the House speaker, Nancy Pelosi, a Democrat of California, warned General Petraeus not to “put a shine” on the battle that kicked off last month for control of Iraq’s second largest city, Basra. Her warning to the general is reminiscent of Moveon.org’s New York Times advertisement last September calling the American commander in the Iraq theater “General Betray Us,” and questioning his integrity. Moveon.org as of last evening was silent on the upcoming testimony.

While early news reports depicted the fighting in Basra as a win for the Iran-based outlaw cleric Moqtada al-Sadr, General Petraeus will likely present a more optimistic assessment.

One source familiar with Multinational Forces Iraq estimates on the battle said that less than 5% of the Iraqi soldiers who fought in the Basra offensive “underperformed.” “They are estimating that 1,200 to 1,500 Iraqi soldiers either defected or went home out of 30,000,” the source, who requested anonymity, said.

The general is also expected to announce his support for a scheduled withdrawal in July of three of the 18 combat brigades in Iraq. General Petraeus will also say that it would be unwise to commit to further troop reductions after July until he and Pentagon leaders have a chance to re-assess the situation on the ground in the summer.

A former senior Iraq adviser to the White House and Coalition Provisional Authority, Dan Senor, said he expects General Petraeus to manage expectations but also report steady progress on security. “His analysis and reporting is typically pretty sober,” Mr. Senor said. “This is working, this isn’t working. I expect in Basra he will try to say the details are still very murky. Here is what we know, and here is the context.”

Mr. Senor, who served as the spokesman for the Coalition Provisional Authority in 2004, said the last time Mr. Sadr attempted an uprising from his bunker in Najaf, the American military had the lead role in fighting in the Shiite holy city, not the fledgling Iraqi military.

General Petraeus will also have an opportunity to visit his family this week and he will visit the Pentagon, according to a senior military official close to the general. He has no plans this week to brief either the president or the defense secretary, whom he briefs weekly by video conference.