WASHINGTON — Americans in a large patchwork of states will file into voting booths and caucuses on Tuesday in the closest thing the country has ever had to a national presidential primary.
The rules differ from state to state and party to party. (Democratic rules provide for delegates to be awarded proportionately on the basis of the popular vote; nine of the Republican contests are winner-take-all. Some contests are not open to independents.) But the combined outcomes could go a long way to determining the nominees, if not in fact effectively ending the races.
On the GOP side, Arizona Sen. John McCain, coming off a crucial Florida win, hopes to roll his current momentum into an irresistible force that overwhelms former Govs. Mitt Romney of Massachusetts and Mike Huckabee of Arkansas.
On the Democratic side, Sens. Hillary Rodham Clinton of New York and Barack Obama of Illinois are looking for momentum-grabbing wins in what is now a two-candidate battle. And both campaigns are aggressively pursuing voters and activists who were aligned with former North Carolina Sen. John Edwards.
Here's a look at some key contests and a glance at some of the latest polling (but surveys aren't elections, and things can change quickly):
GOP: Most-recent poll shows southerner Huckabee with an edge over McCain, and nobody else even close.
DEMOCRATS: Clinton has led in polls for more than a year. But in the most-recent poll, she had only a slight lead over Obama. The state has a large black population, 26.3 percent, which could help Obama.
GOP: Look for big home-state win for McCain.
DEMOCRATS: Most-recent polling shows Clinton leading Obama. This is one of the states that Massachusetts Sen. Edward Kennedy will campaign in for Obama in an effort to appeal to Hispanics, 29 percent of Arizona's population. Obama also has the support of Gov. Janet Napolitano.
GOP: Home-state win is likely on tap for Huckabee, the state's former 10-year governor.
DEMOCRATS: Clinton, the state's former first lady, leads in all polls by margins of at least 20 percent.
GOP: One of the Super Tuesday big kahunas that could help make it a big day for McCain. Gov. Arnold Schwarzenegger's endorsement of McCain will help him.
DEMOCRATS: A new Rasmussen poll shows Obama closing on Clinton. California is another state where Kennedy will be used to appeal to Hispanics, 39 percent of the state's population.
GOP: One of few Super Tuesday states where polls show Romney in the lead.
DEMOCRATS: This is one of the few Super Tuesday states in which Obama has led slightly in recent polls. Colorado is one of the states the Democratic Party is targeting in the fall election, in part because of its 19.7 percent Hispanic population and the Democratic trend in recent state elections.
GOP: Seems headed for big win for McCain, who could get a boost from Rudy Giuliani's endorsement.
DEMOCRATS: Clinton had long led the polls there, but a new Rasmussen poll showed her tied with Obama.
GOP: Huckabee is counting on a Southern win but recent polls show McCain momentum and Romney in striking distance, with Huckabee trailing.
DEMOCRATS: Recent polls show Obama with a lead over Clinton. A Public Policy Polling survey shows Obama getting 73 percent of the state's black vote but trailing Clinton among white voters.
GOP: Heavy Mormon population could help Romney.
DEMOCRATS: The latest polls show Obama with a slight lead over Clinton. But one in five Democrats are undecided, and 15 percent backed Edwards before he dropped out.
GOP: Most recent poll has McCain at 31 percent and Romney second at 20 percent.
DEMOCRATS: Obama has a commanding lead over Clinton in his home state.
DEMOCRATS: Obama's campaign is confident about his chances of winning. He is the son of an African father and a white mother from Kansas. Recently endorsed by Gov. Kathleen Sebelius.
GOP: Should be a home-turf win for former Massachusetts governor Romney.
DEMOCRATS: Kennedy's endorsement of Obama is likely to enhance the Illinois senator's chances. Prior to the Kennedy endorsement, Obama trailed Clinton, but the most recent polls show Clinton's lead shrinking.
GOP: No recent polling data.
DEMOCRATS: Clinton has a comfortable lead in recent polls. But Minnesota is one of the states Obama plans to visit before the voting Tuesday.
GOP: Recent poll gave McCain a lead over Huckabee, who is counting on evangelical Christians.
DEMOCRATS: This is one of the states the Clinton campaign has targeted. Clinton has been leading in polls, but Obama has the backing of Sen. Claire McCaskill, a popular figure in her home state.
GOP: Mid-December poll showed bunched pack with Huckabee, Giuliani, Romney and Thompson, in that order, within four points of each other. McCain was fifth, but expect a reshuffling with Thompson and Giuliani gone.
GOP: A state where Giuliani's withdrawal could have a major impact. Three January polls showed he was a real contender.
DEMOCRATS: Clinton is leading in most polls. She also has the backing of Gov. Jon Corzine and Sen. Bob Menendez. Hispanic voting will be important in New Jersey, where the Latino population is 15.6 percent. But so will the votes of African-Americans, who make up 14.5 percent of the state's population.
DEMOCRATS: New Mexico Gov. Bill Richardson, who served in the Clinton Cabinet, is under pressure to endorse Clinton. Hispanic votes are the key. Forty-four percent of the state's population is Latino.
GOP: Surveys indicate a potential big win for McCain. He led in three of four January polls, with Giuliani second in each.
DEMOCRATS: This is a chance for Clinton to pile up delegates with a big win in her home state. But analysts will look post-Super Tuesday to see how well Obama did in his rival's home state.
GOP: Huckabee is counting on an Oklahoma win. A mid-December poll gave him solid lead on second-place McCain.
DEMOCRATS: Clinton has been the leader in polls for months. In those polls, however, Edwards typically ran third, and winning over his supporters could boost Obama. It also would be a test of how well Obama can do with white voters. The state is 72.1 percent white.
GOP: Former home-state Sen. Thompson had a one-point edge on Huckabee prior to dropping out. Huckabee now is the favorite but recent polls showed McCain momentum.
DEMOCRATS: The latest polls show Clinton leading Obama. Former Vice President Al Gore's endorsement would boost either candidate, but so far, Gore has avoided taking sides, even for the wife of the man he served as vice president.
GOP: Romney is a prohibitive favorite in this heavily Mormon state.
DEMOCRATS: Recent polls show Clinton leading Obama.
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