It’s primary day (again!).

And just because Rick Santorum dropped out of the GOP presidential race doesn’t mean there aren’t plenty of interesting subplots.

Below, we take a look at what you should watch for as results roll in from presidential primaries in Connecticut, Delaware, New York, Pennsylvania and Rhode Island, along with the regular state primary in the Keystone State.

And be sure to stay tuned to The Fix for all the big results Tuesday night.

Does Gingrich drop out?

Maybe the biggest subplot in the presidential race Tuesday night: Keep an eye on the vote in Delaware, the state Newt Gingrich has focused heavily on in the runup to Tuesday’s contests. He suggested to NBC News on Monday that a poor result there might lead him to drop out.

Gingrich arrives at a campaign stop in Buffalo, N.Y., on April 20. (David Duprey — Associated Press)

“I think we need to take a deep look at what we are doing,” Gingrich said. “We will be in North Carolina tomorrow night, and we will look and see what the results are.”

Gingrich set the bar for success, saying he wants to at least come “very, very close” to victory. Of course, he didn’t make a hard-and-fast, in-or-out statement either. He merely said he would “reassess” his campaign.

Then again, the fact that his campaign is more than $4 million in debt gives him plenty of incentive to pack it in.

How does Romney do?

Admittedly, nobody should be reading too much into the results of the GOP presidential primaries Tuesday, but it will still be interesting to see how much of the vote Mitt Romney can get in the five states holding presidential primaries Tuesday night.

All of the states are in his wheelhouse — i.e. the Northeast — so he is supposed to do well anyway. But with Santorum out of the race, people will look to see whether Republicans continue to cast protest votes for the other candidates or rally around Romney now that nearly everyone acknowledges him as the presumptive nominee.

Just remember: A recent Pew survey showed just as much of the GOP base said it would vote for Romney — 90 percent — as the Democratic base said it would vote for Obama.

Which Democratic incumbent emerges in Western Pennsylvania?

Pennsylvania is the only state holding congressional primaries Tuesday, and the big race is between Reps. Jason Altmire and Mark Critz, two western Pennsylvania Democrats who were drawn together thanks to a GOP-drafted redistricting plan.

Altmire looked like the early favorite with much of the merged district (about two-thirds) being his old territory, but Critz has rallied the support of labor unions and is playing up an endorsement from Bill Clinton — two factors that have combined to make this anybody’s ballgame (a Pittsburgh Tribune-Review/WPXI-TV poll last week showed Altmire at 43 percent and Critz at 39 percent).

Unfortunately, there’s really no winner in this primary. Whoever emerges still has to hold a district that went 54 percent for Sen. John McCain (R-Ariz.) in the 2008 presidential race. Waiting in the general election is attorney Keith Rothfus, who fell to Altmire by just two points in 2010 and is running unopposed on the GOP side.

Does Rep. Tim Holden become the latest incumbent (and Blue Dog) to fall?

The loser of Altmire/Critz will be the fourth incumbent to lose this primary season; the question is whether Holden (D-Pa.) becomes the fifth. Republicans drew Holden a much bluer district in redistricting, but it came with 80 percent new territory and a primary challenge for the Blue Dog in the form of attorney Matt Cartwright.

Cartwright is wealthy and has benefited from support from both liberal groups and the anti-incumbent super PAC the Campaign for Primary Accountability, and combined they have all spent more than $1 million against Holden. Cartwright also released a poll recently showing him leading Holden 42 percent to 36 percent.

Holden’s loss would make him the second incumbent to lose to a challenger this primary season, after Rep. Jean Schmidt (R-Ohio), and would further drain the ranks of Blue Dogs in the House — a caucus that is already set to be decimated this year thanks to retirements and redistricting.

Who faces Casey?

Sen. Bob Casey (D-Pa.) isn’t considered one of the most vulnerable senators this year, but this is a state that Republicans have suggested they can win in November, and anything can happen in a swing state.

The GOP primary field is somewhat undistinguished, with coal mine magnate Tom Smith, who has self-funded more than $5 million so far, appearing to be a slight favorite. Also in the mix are former state Rep. Sam Rohrer and businessman Steve Welch. At least if Smith wins, he might force Democrats to expend some resources.

Interesting nugget: Both Smith and Welch used to be Democrats.

Clinton vs. Obama redux

As we discussed last week, this race has stirred memories of the 2008 presidential primary, with Clinton-backed prosecutor Kathleen Kane facing former congressman Patrick Murphy, a 2008 Obama backer who has the endorsement of Obama senior adviser David Axelrod and former governor Ed Rendell.

This has been unusually high-profile for an attorney general primary and it appears to be neck and neck. And if either Murphy or Kane becomes the next attorney general, it’s a sure bet they’ll be looked to for higher office in the coming years. The winner faces Cumberland County District Attorney David Freed (R).

How does Tim Murphy do?

Murphy (R-Pa.) appeared ripe for a primary challenge, but the Club for Growth opted not to get involved, and the CPA, after saying it would spend $200,000 here, backed off. All of that — combined with challenger Evan Feinberg’s slow fundraising pace — suggests Murphy has dodged a bullet. We’ll see for sure Tuesday night.