Hillary Clinton and Barack Obama set out to redefine victory Tuesday, as voters rounded out a day of elections in four critical states that could shape the balance of the race.

Both candidates have put the onus on each other to pull off huge victories Tuesday night, while steering clear of making any bold predictions for themselves. They both also have indicated that Tuesday may not be the end of the line for anyone.

For Obama, victory means maintaining or expanding his 110-delegate lead. Coming off 11 consecutive victories, Obama's campaign claimed that regardless of state-by-state results Clinton will be unable to erode his delegate advantage, making it "virtually impossible" for her to catch up in the remaining contests.

For Clinton, victory means not losing all the contests. Clinton's camp had already argued that Obama needs to sweep the primaries in Texas, Ohio, Vermont and Rhode Island, or face renewed scrutiny from the Democratic Party.

On the GOP side, John McCain was trying to lock down the nomination and shake off his remaining rivals so he can concentrate on the general election.

Polls close in Vermont at 7 p.m. ET, followed by Ohio at 7:30 p.m. ET, and then Texas and Rhode Island at 9 p.m. ET. The four states provide 370 pledged delegates for the Democrats.

"I feel really good about today," Clinton said in Houston Tuesday, urging Texans to participate in the state's two-step, primary-then-caucus process.

Ohio and Texas were once considered must-win for Clinton. But in appearances Tuesday, Clinton sounded like she would continue if she only won Ohio.

"You don't get to the White House as a Democrat without winning Ohio," Clinton said in Houston.

"My husband didn't get the nomination wrapped up until June (in 1992). That has been the tradition," she added, without mentioning that this year most primaries were held much earlier than in 1992.

Recent surveys show a four-contest sweep for either candidate to be unlikely. With Clinton attacking Obama hard on his national security credentials, polls show the race in Texas, which Obama once led, to be in wild flux. Clinton holds a lead in Ohio and Rhode Island, while Obama leads in Vermont.

In San Antonio, Obama called Clinton "a tenacious and determined candidate" and predicted little shift in his delegate lead no matter who won Texas and Ohio, "which means that either way, we'll go on through Mississippi and Wyoming next week." Pennsylvania, the biggest single prize left, follows on April 22.

"All those states coming up are going to make a difference," he said. "What we want to do is make sure we're competing in every single state."

He said he was "confident" he would keep a pledged delegate lead going into the convention, but chided Clinton for running "a pretty negative campaign over the last couple of weeks."

Clinton released an ad in Texas asking voters who they want "answering the phone" in a crisis. She followed up with an ad accusing him of being "too busy" to hold any oversight hearings for a Senate Foreign Relations subcommittee, and consistently argued that the media was being soft on Obama.

"I have said consistently that we do things differently. It's worked for us so far. I'm not going to do things that I'm not comfortable doing," Obama said Tuesday. "This whole spin of just how the press has been tough on them and not tough on us -- I didn't expect that you guys would buy in. It is what it is. I don't want to change the tone of our campaign."

It takes 2,025 delegates to win the Democratic nomination, and slightly more than 600 remained to be picked in the 10 states that vote after Tuesday.

In response to a statement from Obama spokesman Bill Burton claiming it would be "virtually impossible" for Clinton to catch up to Obama in delegates after Tuesday night, Clinton spokesman Howard Wolfson tried to cast Tuesday's contests as symbolic of the candidates' strength on key issues.

"The Democratic Party is not going to nominate someone who fails the commander-in-chief test and the steward-of-the-economy test," Wolfson said.

He went onto to say Tuesday night's results will show Obama "failed that test."

Clinton's campaign has framed Texas as the place for the debate on national security and Ohio as the place for the debate on stewardship of the economy.

Meanwhile, McCain was trying to move ever closer to sealing the nomination.

He began the night with 1,014 delegates, out of the 1,191 needed for the GOP nomination. For Republicans, 256 delegates are at stake Tuesday, so it is possible for McCain to reach 1,191 Tuesday night.

Mike Huckabee had 257 delegates going into the day's contests, and has pledged to tough it out until one candidate reaches the 1,191 threshold.

FOX News' Major Garrett, Bonney Kapp and Aaron Bruns and The Associated Press contributed to this report.