SIMI VALLEY, CALIF. - The presidential nomination battles narrowed to a pair of head-to-head contests Wednesday as Democrat John Edwards and Republican Rudolph Giuliani retreated to the sidelines while the remaining candidates dug in for five days of intensive campaigning before a critical Super Tuesday showdown next week.

Giuliani led the national Republican polls for much of last year, only to see his support plummet in the opening weeks of the primary-caucus season. He folded his campaign Wednesday and immediately endorsed Sen. John McCain of Arizona at a joint news conference.

"John McCain is the most qualified candidate to be the next commander in chief of the United States," Giuliani said with McCain at his side. "He is an American hero, and America could use heroes in the White House. He's a man of honor and integrity, and you can underline both."

Several news services, citing McCain aides, reported that Arnold Schwarzenegger, governor of the crucial Super Tuesday state of California, will endorse the senator today.

Dems face a long, hard fight

On the Democratic side, Edwards, whose angry populism and focus on poverty made him a distinctive voice in the Democratic race, ended his candidacy where it began, in the Lower 9th Ward in New Orleans, which was devastated by Hurricane Katrina.

Edwards said nothing about endorsing either Sen. Hillary Rodham Clinton of New York or Sen. Barack Obama of Illinois, and advisers said he has no imminent plans to do so.

Though the Democratic and Republican races are now essentially two-person contests, their political contours are markedly different. Clinton and Obama are in a frequently nasty personal fight but not one that reflects deep ideological divisions or, as yet, threatens to leave the party badly divided once it is over.

Republicans on the other hand, see the prospect of a clear fracture in their coalition as a result of the nomination contest. McCain is winning important primaries, but he is doing so without the support of the party's conservative or religious base.

Edwards' departure left Clinton and Obama facing a potentially protracted contest that could extend past next week's Super Tuesday to primaries and caucuses stretching into March or beyond. The two Democrats will meet for their first one-on-one debate in Los Angeles at 8 p.m. Thursday night.

During a speech in Denver on Wednesday, Obama used tough language to draw a contrast with Clinton, arguing that she would unite Republicans against her rather than uniting the country and that she would pursue slash-and-burn politics at a time when the public wants to turn away from polarized politics.

Clinton's campaign e-mailed a copy of the Obama speech, labeling it an "angry screed."

Clinton campaigned Wednesday in Arkansas and Georgia. After her landslide loss in South Carolina on Saturday, Clinton focused on black voters, stopping to greet mostly black patrons at a Little Rock diner and then speaking to the National Baptist Conventions in Atlanta in the late afternoon.

McCain could take charge

The Republican race could reach an effective conclusion in Tuesday's balloting, with McCain, coming off his victory in Florida on Tuesday, determined to close out the challenge from former Massachusetts governor Mitt Romney.

Former Arkansas governor Mike Huckabee, who finished fourth in Florida, still poses a potential obstacle to Romney, especially in many of the Southern primaries on Tuesday. Rep. Ron Paul of Texas, meanwhile, has struggled to expand his support beyond a dogged but small anti-establishment constituency.

But Romney advisers see an opportunity to draw a series of bright distinctions that will highlight ideological differences between the McCain and Romney as well as casting the race as outsider versus insider and future versus past.

"That is the contrast and over the next seven days we have to make that case," said Kevin Madden, Romney's press secretary.