WASHINGTON, June 28 ? Standing on a stretch of land northwest of the Capitol on a recent afternoon, Representative John Lewis took a chest-swelling breath and surveyed the space around him ? a 5.25-acre expanse dotted with sweet gum and elm trees, park benches and a temporary parking lot.

"It would be very powerful," Mr. Lewis said with a pensive sigh. "The healing we could foster. The message we could send from here."

That morning Senator Sam Brownback, a Republican from Kansas who shares little political ground with Mr. Lewis, a liberal Democrat from Georgia, jogged by the same piece of land. "Just passing it," he said. "Filled me with a sense of hope."

On the same day, Robert Wilkins, a tenacious 39-year-old lawyer, sat in his downtown office with detailed sketches of the ground. The space, and the struggle it symbolized, have "become an obsession of sorts," he said.

The civil rights stalwart, the conservative Republican and the impassioned lawyer represent a coalition of allies in a nearly century-long quest to build a national museum of African-American history and culture here on the National Mall. And in an improbable alignment of the social and political stars, proponents of the museum say they believe the project may finally come to fruition, given a crucial push from President Bush and a Republican-controlled Congress.

A presidential commission, sanctioned in late 2001, conducted a feasibility study for the museum. The commission's report advocating a museum formed the basis for legislation that the Senate passed by unanimous vote on June 23. The House is expected to pass the measure before its August recess.

The land, in the shadow of the Capitol, though just one of four proposed sites, is favored by most of the museum's supporters. Recently uncovered historical documents show that it was there that a group of black Civil War veterans gathered in 1915 to rally for a memorial honoring black contributions to the country.

Republican support for the current effort has raised the eyebrows of some supporters of the museum, who question whether it is a calculated effort to court black voters before next year's presidential election. But Mr. Lewis, who has introduced bills for the museum every year since 1988, takes a more pragmatic view.

"It took Nixon to go to China," he said. "Sometimes it takes a blending of political forces. I just want to see it done."

Congressional approval is the first of many hurdles. Though the Senate authorized $17 million in startup money for 2004, consultants on the project estimate that the museum will take several years and more than $300 million to build. Choosing the museum's leadership could prove contentious, as could shaping the content and tone of its exhibits.

The museum would cover a sweeping history from the Middle Passage to the modern black diaspora. Detractors have long expressed concern about how it would deal with slavery and how it would portray the South.

However painful the subject, supporters argue that the project is long overdue. The United States Holocaust Memorial Museum is celebrating its 10th anniversary this year. The National Museum of the American Indian is to open on the Mall next year.

"You cannot tell American history without telling African-American history," said Claudine Brown, vice chairwoman of the presidential commission.