Tuesday looms as a crucial day in the U.S. presidential race as Democrats Barack Obama and Hillary Clinton battle in four states holding primaries. Meanwhile, Republican John McCain is on the verge of wrapping up his party's presidential nomination. VOA National Correspondent Jim Malone reports on the election campaign from Washington.

Ohio, Texas, Vermont and Rhode Island hold primaries Tuesday in what could be a decisive showdown in the race for the Democratic nomination.

Hillary Clinton is pinning her hopes of staying in the race on winning the two largest states, Ohio and Texas.

The latest polls show Clinton leading in Ohio, while Obama may have a slight edge in Texas.

Foreign policy and national security have emerged as key issues in recent days.

The Clinton campaign is running a television ad questioning whether Obama is ready to handle a crisis overseas as president.

"It is 3:00 am and your children are safe and asleep. But there is a phone in the White House and it is ringing. Something is happening in the world. Your vote will decide who answers that call."

Clinton rallied supporters in Ohio by emphasizing her experience as a senator and as First Lady during her husband's administration.

"Who would you hire to end the war in Iraq and answer the phone at 3:00 am in the White House? I promise you, I will work my heart out for you," she said. "I hope you will come out and vote for me tomorrow! Let us go make history! Thank you!"

During his campaign stops in Texas and Ohio, Obama has countered that Clinton's initial support for the war in Iraq undermines her claim of better experience and judgment on foreign policy.

"To this day, she will not even admit that her vote was a mistake or that it was even a vote for war," he said. "So besides the decision to invade Iraq, we are still waiting to hear Senator Clinton tell us what precise foreign policy experience that she is claiming."

Obama leads in delegates for the party nomination and has won 11 straight contests against Clinton.

The latest estimate from the Associated Press shows Obama has nearly 1,400 committed delegates, while Clinton has nearly 1,300. To win the Democratic nomination 2,025 are needed.

Democrats award delegates on a proportional basis, which means it will be difficult for Clinton to catch up to Obama unless she wins individual primary and caucus contests by wide margins.

At least one prominent Democrat believes his party should be willing to close ranks behind one of the two remaining contenders following Tuesday's primaries. New Mexico Governor Bill Richardson told the CBS program Face the Nation the party should rally around whichever candidate has more delegates after Tuesday's voting.

"We have to have a positive campaign after Tuesday," he said. "Whoever has the most delegates after Tuesday, a clear lead, should be in my judgment, the nominee. I think we have to be ready for a very strong John McCain. Republicans are united right now."

Richardson withdrew from the Democratic race in January, but has yet to endorse either Obama or Clinton.

In the Republican race, Senator John McCain is in a position to effectively clinch the party nomination with victories in the four states holding primaries on Tuesday.

McCain is within striking distance of the 1,191 delegates needed to secure the nomination against two remaining Republican challengers, former Arkansas governor Mike Huckabee and Texas Congressman Ron Paul.