PROVIDENCE, Rhode Island (CNN) -- Tuesday could be the most consequential day to date in the race to the White House. There's a good chance voters in Texas, Ohio, Rhode Island and Vermont will make the difference in deciding the Democratic and Republican presidential nominees.
Sen. Barack Obama and his rival have been focusing on the economy and trade in Ohio.
The results of those four states could mathematically put Sen. John McCain of Arizona over the top when it comes to winning enough delegates to secure the GOP nomination. They also could determine whether the fight between Sens. Hillary Clinton of New York and Barack Obama of Illinois for the Democratic nomination ends this week or continues into the spring.
Obama comes into the March 4 primaries with momentum on his side, thanks to his 11 straight victories in contests dating back nearly a month. He has 1,378 pledged delegates and superdelegates to Clinton's 1,269.
Neither candidate is close to the 2,025 needed to win the Democratic nomination.
The big question surrounding Clinton is how many states she needs to win Tuesday to keep going.
Former President Bill Clinton said in February if his wife wins Ohio and Texas, she'll go on to win the nomination. The flip side, of course, is that if she doesn't win those two big states, will she give up her quest for the White House?
A lot of people inside and outside the Clinton campaign have weighed in on this question, but the one voice that matters is staying silent. Asked this weekend if she had to win Ohio and Texas, Clinton wouldn't answer other than to say, "I never make predictions."
The stakes Tuesday add to the political suspense.
"If Obama wins Texas and Ohio, it's game over," said CNN senior political analyst Bill Schneider. "If Clinton wins Texas and Ohio, it's game on until someone can figure out how to reach a majority of delegates. That may not happen until the Democratic National Convention in late summer. If Clinton and Obama split Texas and Ohio, it's a new game."
Texas and Ohio are the biggest prizes Tuesday -- 193 Democratic delegates are at stake in Texas and 141 in Ohio. It's a political rush hour in those states right now, as they get most of the attention from the candidates, their campaigns and the media. Watch Clinton campaign in Ohio Monday »
CNN's poll of polls, an averaging of the most recent surveys in each state, suggests the race is extremely tight, with Obama ahead by 2 points in Texas and Clinton ahead by 5 in Ohio. But the polls also indicate there are still many undecided voters in both states.
Texas was supposed to be Clinton country, because of its large Latino population, a group Clinton has done well with so far this primary season. But Texas also has a strong African-American population, and Obama has dominated the black vote so far this year. Watch CNN's roundup of the March 4 contests »
A breakdown of a recent CNN/Opinion Research Corp. poll in Texas of how whites, African-Americans and Latinos are planning to vote suggests why the race there is so tight.
"Obama appears to be picking up support from nearly 8 in 10 blacks," said CNN polling director Keating Holland. "Clinton may win roughly two-thirds of the Latino vote. There are likely to be more Latino voters than blacks when Democrats go to the polls on March 4, which should work to Clinton's advantage.
"But Obama's huge lead among blacks -- plus a noticeable chunk of the Latino vote -- tends to counteract that. The result: Clinton and Obama get roughly equal numbers of votes from non-white Democrats. And since whites appear to be splitting almost evenly between the two candidates, the overall race is a virtual tie."
Some people in Texas get to vote twice, and that could help Obama. In Texas, 126 of the delegates at stake are decided in the primary, but once the polls close Tuesday night, Texas also holds caucuses, where those who cast ballots in the primary get to vote again. Another 67 delegates will be allocated in those caucuses. Obama has done extremely well in states with caucuses. Watch how Texas residents can vote twice »
In Ohio, Clinton has a slight lead, because of the large number of union and blue-collar workers and Catholic voters, groups that, along with women and senior citizens, are considered her base.
Rhode Island, which has 21 Democratic delegates, is in some ways a smaller-scale Ohio, thanks to its large Catholic and working-class population, and Clinton is ahead in the polls there. Watch more about the Rhode Island primaries »
Vermont is a different story, with many upscale voters and liberals and few industrial workers. Obama leads in the polls there by more than 20 points.
For the Republicans, it is not a question of who but when. Sen. John McCain of Arizona has 1,047 delegates, 144 shy of the 1,191 he needs to lock in the Republican nomination. On Tuesday, 256 GOP delegates are at stake, so if McCain has a good night, he can go over the top.
Challenger Mike Huckabee is still in the race, even though he faces astronomical odds. The former Arkansas governor has said he would bow out if McCain wins a majority of the delegates. E-mail to a friend