Tuesday's Republican primaries could be a final nail in the coffin for the rivals of leading candidate Mitt Romney, making it impossible for them to catch up as he has all but won the nomination to challenge U.S. President Barack Obama in November's elections, experts said.

The former Massachusetts governor is expected to sweep all five GOP primaries in Delaware, Rhode Island, New York, Connecticut and Pennsylvania -- where a total of 209 delegates will go to the winner -- surging even further ahead toward the 1,144 delegates he needs to get his party's nomination.

That will be even easier, experts said, after second-placed former Pennsylvania Senator Rick Santorum threw in the towel earlier this month.

"For all practical purposes, Romney is the GOP nominee and there is little (Newt) Gingrich or (Ron) Paul can do about that," said Darrell West, vice president and director of governance studies at the Brookings Institution. "Neither opponent is putting much effort into campaigning at this point so their continuing campaigns are more symbolic than substantive."

"Soon they will acknowledge this reality and suspend their campaigns," West added.

Up to this point, opponents Gingrich and Paul have held on for dear life in a race in which it has become increasingly clear that Romney will emerge as the victor.

The most recent U.S. media count has Romney at 697 delegates. Gingrich trails behind at 137 and Paul has 67. With the primary process past the halfway mark, catching up to Romney could be a numerical impossibility after Tuesday, noted the TV network Fox News.

History also bodes ill for Paul and Gingrich. There is no previous example of a candidate catching up at this point in the primaries, although there have been a couple of close calls.

In 1976, former President Gerald Ford won almost all of the early Republican contests, but Ronald Reagan came out swinging and nearly caught Ford in total delegates before Ford came back. And in 1972, Ed Muskie won in the early states before falling out of serious contention in the Democratic primaries.

CAN ROMNEY BEAT OBAMA?

The latest polls have Obama and Romney neck-in-neck, with the president leading by only three points, according to the most recent Real Clear Politics averages.

Romney will capitalize on the weak economy and try to peel away former Obama supporters who have become dissatisfied with the president, said Ryan Prucker, president of Imagelight/Personality Driven Media, a media consulting company.

But the race will come down not only to the economy, but also to how well each candidate can convince voters that he is the man for the job.

And that's where Romney will have to really state his case about why he believes he has a better vision for the future. But because he's not a great communicator, it will be a stumbling block for him to articulate -- he is often seen as a robot-like persona who has trouble connecting to voters, Prucker said.

Meanwhile, President Obama on Tuesday kicked off a mini-tour of college campuses in North Carolina, Iowa and Colorado -- three swing states crucial to the November elections -- to talk about student loans and educational costs in a bid to court the crucial youth vote.

The problem is not support for the president among young people- - Obama in 2008 clinched two-thirds of an under-30 vote that today remains Democratic -- but rather to energize young voters to show up at the polls amid high unemployment, the rising cost of a college degree and surging prices at the gas pump.