WASHINGTON -- Hillary Rodham Clinton and Barack Obama were jostling for the lead going into Tuesday's coast-to-coast presidential primary contests, while Republican frontrunner John McCain aimed to lock down his party's nomination by squeezing main rival Mitt Romney out of the race. While McCain appeared to have a substantial lead in polls on the Republican side, the Democratic race was far from clear.

One national poll shows senators Clinton and Obama locked in a historic race for the party's nomination, tied going into Democrats' 22 contests on Tuesday, while another shows Clinton with a slight lead. A poll released Monday even showed Obama with a slight lead in California and Missouri.

The polls may vary, but they show how Obama, who would be the U.S. first black president, has closed in on Clinton's once strong lead.

On Sunday, Obama and Clinton were courting each other's core constituencies -- black voters and women, respectively -- as they rallied for an edge.

Meanwhile, McCain was looking to beat Romney on his home turf. On Monday he was taking his campaign to Massachusetts, a state where Romney served as governor, as he looked to knock out the millionaire former businessman on Super Tuesday when Republican contests are held in 21 states.

The Arizona senator looks to do well among moderate northeastern Republicans after his victory in Florida's primary last Tuesday led former New York Mayor Rudy Giuliani to quit the race and endorse McCain.

Gone from McCain's recent campaign stops was a mention of Romney, who McCain recently has lumped in with the Democrats on the question of when U.S. troops should leave Iraq.

"The first thing we've got to do after Tuesday is unite this party," McCain says repeatedly these days -- as if the 21 states holding caucuses and primaries this week are simply a formality.

Meanwhile, Romney continued to hammer away at his assertion that McCain is weak on economic issues and is too much of a maverick for the conservative party. "If we want a party that is indistinguishable from Hillary Clinton on an issue like illegal immigration, we're going to have John McCain as a nominee. That's the wrong way to go," Romney said.

McCain said on a news talk show Sunday that he is "far more conservative" than Romney.

The Democrats on Sunday were relying on their highest profile surrogates to woo voters in California -- the big Super Tuesday prize with its 370 delegates. In that state, a survey of California Democrats showed Clinton had 36 percent, Obama 34 percent, with 18 percent undecided. The Field Research Corp. poll, conducted Jan. 25-Feb. 1 had a margin of error of 4.5 percentage points.

A Reuters/C-SPAN/Zogby poll released Monday showed Obama leading with 46 percent to Clinton's 40 percent in California, and ahead with 47 percent to Clinton's 42 percent in Missouri, Reuters reported Monday. The polls had margins of error of 3.2 percentage points in California and 3.4 percentage points in Missouri.

Obama's wife, Michelle, TV talk show host Oprah Winfrey, and Caroline Kennedy, daughter of slain President John F. Kennedy, led a rally in Los Angeles on Sunday. Their joint appearance targeted female Democrats who tend to favor Clinton, who is seeking to become the first woman U.S. president.

Former President Bill Clinton also spent the day in Los Angeles visiting four churches in mostly black neighborhoods. The trip was widely seen as a bid to smooth over perceptions that he had injected race into last month's Democratic primary in South Carolina, which Obama won handily.

The former president never mentioned Obama by name when he spoke for about 20 minutes at the City of Refuge church in Gardena. But he struck a conciliatory tone in describing this year's Democratic contest as "an embarrassment of riches."

Hillary Clinton spoke at the Greater Mount Carmel Missionary Baptist Church in St. Louis, saying it "was a great moment" for the party and America when a woman and a black man emerged as the two remaining contenders for the Democratic nomination.

Careful not to criticize Obama, Clinton urged the black churchgoers to consider her background in Arkansas, in the White House as first lady and in the Senate.

With McCain emerging as the likely Republican nominee, Obama and Clinton argued in Sunday television interviews over who would have the best chance of defeating him in the November election.

Obama, speaking on CBS's "Face The Nation" before campaigning in Wilmington, Delaware, said Republicans and independents would be more inclined to support him than Clinton in the November general election.

The problem is "not all of Senator Clinton's making," he said, "but I don't think there's any doubt that the Republicans consider her a polarizing figure."

But Clinton suggested that Obama, who was elected to the Senate in 2004, would be more susceptible because he has not been scrutinized for years as she has. "I've been through the Republican attacks over and over again," she said on ABC's "This Week."

Meanwhile, the third-place Republican candidate, former Arkansas Gov. Mike Huckabee, was concentrating on the South, with appearances in Georgia and Tennessee.

Among Democrats, a new nationwide poll by the Pew Research Center showed Clinton at 46 percent and Obama at 38 percent, with the number of undecided voters increasing to 15 percent. Clinton had led by 15 percentage points in mid-January. Former Sen. John Edwards quit the race on Wednesday.

The same poll showed McCain leading among Republicans with 42 percent, while Romney trailed with 22 percent and Huckabee had 20 percent. McCain is up 13 percentage points since mid-January.

The poll was conducted from Jan. 30-Feb. 2, with a margin of error of 4.5 percentage points for Democrats and 5 percentage points for Republicans.

McCain is now favored to win Connecticut, New York, New Jersey, Connecticut and Delaware, which award all their convention delegates to the top vote-getter. That would give him 198 delegates toward the 1,191 needed to secure the nomination at the party's national convention this summer.

McCain is also leading in polls of several southern states because Romney and Huckabee are splitting the votes of conservatives opposed to McCain.

McCain could emerge as the party's presumptive nominee after Super Tuesday because nine of the 21 Republican contests award delegates on a winner-take-all basis to the top vote-getter.

The Republicans have 1,023 delegates at stake on Tuesday. McCain leads with 93 delegates, followed by Romney (77), Huckabee (40) and Texas Rep. Ron Paul (4), according to the latest AP tally.

A total of 1,681 delegates are at stake for Obama and Clinton in the 22 Democratic races on Tuesday, but the Democrats award their delegates proportionally. In all, 2,025 delegates are needed to win the Democratic nomination, and the two campaigns have said they do not expect either candidate to lock in the nomination that day. The AP puts Clinton's delegate tally at 261 while Obama has 190.