Good Tuesday morning. Iowa was a big day, New Hampshire was even bigger, and Super Tuesday was huge. But today is like Christmas and D-Day rolled into one. After six weeks of anticipation, it's finally Earth Day. Oh, and some voters in Pennsylvania are also voting on something or other, we're not sure. Here's what Washington is watching today:

-- The Senate takes up a bill on insurance reforms for disabled veterans. Later this afternoon, the chamber will recess for the unveiling of a portrait of former Majority Leader Tom Daschle, the South Dakota Democrat who lost his seat in 2004. The House takes up a series of bills under suspension of the rules, including a bill on Medicaid fixes. Later, the chamber takes up Copper River salmon (which any seafood lover will tell you is the best kind of salmon) and resolutions expressing recognition of Earth Day, National Health Care Decisions Day, World Glaucoma Day and a celebration of Israel's 60th birthday. President Bush is in New Orleans meeting with Canadian Prime Minister Harper and Mexican President Calderon for a summit, then hosts a lunch with community leaders, plants a tree and fundraises for Senate candidate John Kennedy, the Republican with the best shot at knocking off an incumbent Democrat.

-- But of course, all eyes will be on Pennsylvania, where 158 delegates are at stake in today's presidential primary. Polls close tonight at 8 p.m. (check out live-blogging coverage over at The RCP Blog) across the Keystone State in yet another must-win contest for Hillary Clinton. Her options: Win big, stay in the race. Win small, hear mounting calls for an end to the race and probably a dozen or so super delegates who flock to Barack Obama's campaign in a very public manner in the next few days. Lose and get out of the race. The latest RCP Pennsylvania Average -- which could be updated throughout the day as new polls roll in -- has Clinton leading by six points, but of course the real question is what constitutes a win for each candidate.

-- A win for Clinton: Her advisers are spinning it simply; a win is a win is a win. But Clinton needs to surprise some people tonight with her margin of victory to give her a counterargument when the Obama team points out that she failed to win back enough delegates to seriously cut into his lead. A surprise is not out of the question, and Clinton did earn a huge boost after wins in Ohio, Texas and Rhode Island. Clinton faces one more challenge today than she did back then, though: Her campaign is virtually broke, owing nearly as much to Mark Penn's polling firm as it has available for primary contributions. Clinton, in short, needs a big win for both the money and the momentum, without which not only will the calls for her to get out of the race mount, but she simply won't be able to compete in Indiana and North Carolina, or anywhere else, for that matter.

-- A win for Obama: Chief strategist David Axelrod has been telling reporters that they need to keep Clinton's margin under ten points, and that will be good enough to be considered a win. Huge turnout -- more than 215,000 new Democrats registered across Philadelphia alone -- could give him a boost, and the Obama campaign has sent in some powerful and experienced veterans to work turnout in Philadelphia. The city is about half African American, and a big turnout there will help Obama close Clinton margins elsewhere in the state. But given that Obama's campaign has actively worked so hard to win Pennsylvania, the race should be closer than it is, meaning a real Obama win should be within reach, if not expected. After all, you don't spend $10 million on television ads if your goal is just to lose by less than ten.

-- Democrats are likely to turn out in droves today, and more than half of the 4.2 million Democrats registered in the state are expected to go pull a lever. The Department of Justice is on hand in Philadelphia to keep an eye on accommodations made for Spanish-speaking voters, the Philadelphia Inquirer reports, and poll workers are prepared to be overwhelmed. How they handle the mad rush is anybody's guess, but here's betting that, as in other states, some stations will run out of ballots, both campaigns will cry foul and someone will be unhappily convinced that the results reported tonight aren't what they should have been.

-- Going forward, a new debate ahead of North Carolina's May 6 primary is not going to happen, the North Carolina Democratic Party announced yesterday, per the Charlotte Observer. The state party, along with outgoing Governor Mike Easley and other top Democrats, had been working with CBS to schedule an April 27 debate in Raleigh that would have given Katie Couric her one shot to moderate a Democratic throw-down. It would have meant a $300,000 boost to the party organization, one that is going to have to defend Easley's seat and gun for Republican Senator Elizabeth Dole in November. Still, both candidates are scheduled to address the Jefferson Jackson Dinner in Raleigh next Friday.

-- Who's responsible for the death of the debate? That would be the Obama campaign. Some supporters of the Illinois Senator even chanted "No more debates" at a Raleigh get-together this week. It's a smart strategy as well; if a debate is on the calendar, Clinton, despite a worse than expected showing in Pennsylvania, would have reason to stay in the race, and at this point she's looking for all the reason she can find. After the debate takes place, then the justification for staying in through the May 6 primaries in North Carolina and Indiana is there. Even if she doesn't win big in both states, West Virginia is the following Tuesday, and Clinton is expected to do very well there. One step leads to another, and soon the Democratic race could be into June.

-- Trend Of The Day: In virtually every primary contest so far, a stark trend has emerged: The older a voter is, the more likely they are to vote for Hillary Clinton. The younger a voter is, the more likely they are to vote for Barack Obama, as the New York Times' Katharine Seelye writes today. Pennsylvania is one of the oldest states in the nation -- the average Pennsylvanian is nearly three years older than the national average, and 58% of registered Democrats are older than 45. Imagine an Obama-McCain matchup in November, when the differences between the two is more generational than even the differences between the two Democrats. Obama will likely have to rely on the youth vote all the more, a daunting and dangerous prospect, but one he's achieved on a number of occasions so far.

-- Today On The Trail: Clinton and Obama are hitting the morning shows, when taped interviews aired on ABC, NBC and CBS. Neither has public events today, but this evening both will be watching election results with fans and supporters. Clinton will be at a Philadelphia hotel, while Obama will be at a concert John Mellencamp is putting on for supporters in Evansville, Indiana. Completely neglected today, John McCain will continue his forgotten America tour with a speech on the economy at a company in Youngstown, Ohio, followed by a town hall meeting at Youngstown State University and a media availability.