WASHINGTON (CNN) -- Four states -- Ohio, Rhode Island, Texas and Vermont -- hold contests on Tuesday that could be make-or-break for both parties' presidential hopefuls.
Early voting began February 19 across Texas ahead of the state's primary.
At stake on Tuesday: 370 Democratic and 256 GOP delegates.
On the Republican side, the primaries could give Arizona Sen. John McCain enough delegates to claim the GOP nomination.
For Democrats, Sen. Hillary Clinton of New York is looking to stop Sen. Barack Obama's winning streak of 11 straight contests since Super Tuesday and breathe new life into her campaign
As of Monday, Obama has 1,369 delegates (pledged: 1,184 superdelegates: 185). Clinton, meanwhile, has 1,267 delegates (pledged: 1,031, superdelegates: 236). To win the nomination a candidate needs 2,025 delegates.
Ohio, a long-time bellwether state, could prove crucial to Clinton staying in the race. Her husband, former President Bill Clinton, has said she must win in Texas to keep her campaign going.
Trade and the economy will likely decide who Ohio Democrats vote for.
In Texas, Latinos -- a strong voting bloc for Clinton -- could prove crucial. Obama has been cutting away at that base of support for Clinton in recent contests.
Clinton and Obama are in a statistical dead heat in Texas, according to a poll released on February 25.
McCain currently has 1,033 total delegates (pledged: 967, unpledged RNC: 66).
Most of the 141 Democratic delegates at stake will be awarded proportionally based on congressional districts.
The primary is open, meaning a registered voter may participate in either the Democratic or Republican primaries, but not both.
In CNN's Ohio Democratic "poll of polls" of likely Democratic voters released Monday, Clinton edges out Obama slightly: 48 percent to 43 percent for the Illinois senator. The polls showed 9 percent unsure.
The polls were taken from February 26 to March 2.
On the Republican side, the 84 GOP delegates at stake will go to the candidate who wins a statewide majority of the votes. Like Democrats, the primary is open.
This crucial presidential battleground has few immigrants and many military veterans, more than 13 percent, according to the Almanac of American Politics. Watch an analysis of the showdown in Ohio »
Ohio's Republican Party has recently suffered from a series of political scandals, allowing Democrats to win the governor's seat and to gain key seats in Congress.
In the northeastern part of the state, heavy industry fostered a strong labor union following, while a more diversified economy has made southwestern Ohio less dependent on manufacturing.
This could provide a crucial factor for Clinton, who tends to do well with the so-called "lunch-box Democrats" -- a group made up of working-class individuals.
However, in the past three contests, Obama has been cutting into her lead with that demographic.
And for Democrats, turnout will be key -- something that is already breaking records.
In the 2000 presidential primary campaign, 10,371 absentee ballots were requested from voters in both parties. Four years later, there were 9,600 requests.
This year, there are more than 40,000, just in the Cincinnati area, which is part of an unprecedented early and absentee voting pattern across the state.
Ohio, it appears, will be no exception in a presidential primary season punctuated by remarkable Democratic intensity and some signs of a shrinking or changing Republican base.
"More people are telling us they are going to be voting in the Democratic primary," said Eric Rademacher, associate director of the Institute for Policy Research at the University of Cincinnati.
"And when we look at our polls over time, we are seeing a little bit of a dip in the number of people who are self identifying as Republicans."
By the numbers, Republicans have a serious case of turnout trouble.
Excluding caucuses, some 22 million Democratic votes have been cast in the primaries held to date. For Republicans, the number is 14.1 million.
One reason Republicans cite is that the Democratic contest is highly competitive while the Republican race is all but over.
But GOP turnout has been down since the beginning of the year -- even when the Republican race was wide open.
Texans head to the polls for the state's unique primary/caucuses, which allot many delegates based on voter turnout in 2004.
This might prompt candidates to focus on areas with higher populations.
About a third of Texas' residents described themselves as Hispanic in 2000, a quickly growing voter bloc prized by both parties. Clinton is expected to do well in low-turnout Latino districts.
In Texas, as in other states, Clinton does best with older, blue-collar and rural voters.
Clinton also boasts a strong political network in the state where she and her husband, Bill Clinton, campaigned for Democratic candidate George McGovern in 1972.
As the fight rages on over the precious delegates, the Clinton camp has described Texas and Ohio as her "firewall" aimed at stopping Obama's recent momentum.
Texas' complicated system of choosing delegates uses a combination of primaries and caucuses on March 4. Watch an explanation of the confusing process »
Most of the 193 Democratic delegates at stake will be allocated proportionally by open primary elections in each district. But other Democratic delegates will be chosen by attendees at precinct caucuses that begin after polls close.
And from CNN's Texas "poll of polls" released Monday, the race is very close. Obama leads with 47 percent to Clinton's 45 percent. The polls showed 8 percent unsure.
The polls were taken February 26 through March 2.
The GOP primary, meanwhile, chooses most of the 137 delegates at stake based on proportional results in each congressional district. A lesser number of at-large delegates will be allocated by the statewide primary results.
Candidates take all delegates in a district if the candidate receives more than 50 percent of the votes. Watch as CNN's Ted Rowlands takes a look at Texas GOP voters »
Rhode Islanders also head to the polls on March 4.
The Democratic and Republican primaries are open. That means they are open to registered party members and independents who must affiliate with a party at the polls, but may change back to independent after voting.
There are 21 Democratic delegates at stake, which are allocated proportionally statewide and by congressional district.
Republicans have 17 delegates at stake, allocated proportionally by a congressional district vote.
In Vermont, both Democrats and Republicans vote in the state's primaries.
The Democratic primary is open, so any registered voter may participate in either primary.
At stake are 15 delegates, allocated proportionally statewide and by congressional district.
And for Republicans, the open primary could benefit McCain who does well with Independent voters.
There are 17 delegates at stake, allocated in a winner-take-all method.
The Democratic Texas "poll of polls" consists of five surveys: Reuters/C-SPAN/Houston Chronicle/Zogby (February 29-March 2), Belo/Public Strategies (February 28-March 1), American Research Group (February 29-March 1), MSNBC/McClatchy/Ft. Worth Star-Telegram/Mason-Dixon (February 27-29), and Fox News/Opinion Dynamics (February 26-28)
The Democratic Ohio "poll of polls" consists of six surveys: Reuters/CSPAN/Houston Chronicle/Zogby (February 29-March 2), Quinnipiac (February 27-March 2), University of Cincinnati "Ohio Poll" (February 28-March 2), American Research Group (February 29-March 1), Cleveland Plain Dealer/Mason-Dixon (February 27-29), and Fox News/Opinion Dynamics (February 26-28) E-mail to a friend
CNN's John King, Bill Schneider, Paul Steinhauser and Robert Yoon contributed to this report.