When 22 states selected Democratic delegates on a single day last month, the sheer scale made election night returns difficult to follow.
The voting on Tuesday could be decisive for the race between Senators and . With four states voting —, , and —there is less to keep track of.
But with so much at stake, especially in Ohio and Texas, and with the proceedings in Texas more complicated than a simple primary, there will be no shortage of facts and analysis as the evening unfolds.
There are some factors to watch for.
A result virtually everyone in presidential politics will scrutinize most intently is whether the results prod Mrs. Clinton to end her effort to become the first female president.
The more decisive the outcome, the easier her choice. Should Mr. Obama sweep all four contests, her hopes will plainly be extinguished.
Should she carry Ohio and Texas — as her husband, the former president, said she must to retain a shot at the nomination — she will no doubt fight on to the next big battle, on April 22 in Pennsylvania, and, perhaps, all the way to the convention in Denver.
Trickier to handicap would be a split decision, in which Mrs. Clinton won Ohio, for example, but lost Texas.
For people who want to think about possible permutations, keep this one in mind. Because of the way Texas allocates delegates, it is entirely possible that Mrs. Clinton could win the popular vote there but lose to Mr. Obama on delegates.
Out of the gate
In many elections, the first precincts reporting vote totals do not mean much, because the totals bear little relation to the ultimate outcome. But the first returns in Texas, where all polls will close by 9 p.m. Eastern time, may count a lot.
That is because they will represent “early voting” from before Election Day, a segment that the Clinton campaign considers crucial to its hopes. Early voting could make up one-third of the turnout.
Allies of Clinton
For much of the primary campaign, demography has been destiny. So Mrs. Clinton’s hopes in Ohio and Texas could turn on the turnout composition.
In Texas, Mrs. Clinton wants the Hispanic constituency that has favored her to loom as large as possible. Strategists in both campaigns say it could be 40 percent of the vote.
In Ohio, the key is how much women dominate the electorate. Women have been a majority of Democratic primary voters. The more their share exceeds 55 percent, the better for Mrs. Clinton.
Mrs. Clinton’s campaign is also focusing Democrats in relatively conservative southern Ohio, the home region of her top ally in the state, Gov. Ted Strickland.
Keys for Obama
Critical constituencies for Mr. Obama include young voters and African-Americans. One remarkable feature of his Iowa and South Carolina victories was that voters younger than 30 turned out as heavily as those 65 and older.
Mr. Obama is counting on heavy turnout in university towns like Austin and Columbus.
Because of the apportioning of delegates, African-Americans in Ohio and Texas could matter more than their estimated 15 percent share of the electorate would suggest.
That is why urban centers like Cleveland and Houston are central to his strategy. His strategists are also watching blue-collars towns like Akron and Youngstown to gauge their success at peeling working-class voters, especially men, away from Mrs. Clinton.
With a long track record in Democratic politics, Mrs. Clinton prefers contests limited to party regulars. Mr. Obama has held the upper hand among independents.
Independents can vote in all four primaries on Tuesday. Experts do not expect the reach their level in New Hampshire, more than 40 percent.
Texas and Vermont have no party registration. Ohio and Rhode Island let independents in either primary.
Although big states dominate the headlines, small ones sometimes have an outsize role in the count for nominating delegates. Mr. Obama leads Mrs. Clinton by roughly 150 delegates, depending on who is counting. He remains far short of the 2,025 needed for the nomination.
Because Democrats allocate delegates in primaries in proportion to the popular vote, close contests sometimes leave the candidate who finishes second in the popular vote with virtually the same number of delegates as the winner.
Tiny Vermont, with one-tenth the delegates at stake in Texas, could have a larger effect on the delegate race by adding as many as five to Mr. Obama’s lead if he wins by a large margin there.
Working the process
Mr. Obama, a one-time community organizer, has dominated caucus contests that place a premium on political mechanics. The good news for Mrs. Clinton is that the four states have primaries. The bad news is the Texas Two-Step, which will include evening caucuses to select one-third of the delegates. Though Mrs. Clinton is competing more aggressively in those caucuses than in some earlier states, a chance remains that she could win the primary vote but lose enough delegates in the caucuses as to cast doubt on who “won” the state.
The more caucus turnout increases, the greater the challenge for Texas Democrats to administer a process that insiders usually dominate. Ohio officials, criticized for their oversight of balloting in the 2004 presidential contest, will face similar tests if turnout surges as it has in earlier primaries.
Putting on a good face
Campaigns always like as much attention as possible for their victories. Sometimes how quickly the news media report them determines that.
Pre-election polls point to a lopsided race in the state with the earliest poll-closing time, Vermont. That could produce good news for Mr. Obama soon after its polls close at 7 p.m. Polls in Ohio, where Mrs. Clinton has led, close 30 minutes later. Rhode Islanders will vote until 9 p.m. Eastern time.