WASHINGTON – It's official: Democrats have a primary in Pennsylvania's critical U.S. Senate race.

Katie McGinty, former chief of staff to Gov. Wolf, jumped into the campaign Tuesday in a move that changes the political outlook for the nine or so months leading up to next year's Democratic primary. She'll face a big challenge in trying to top former Delaware County Congressman Joe Sestak as they vie to take on Republican Sen. Pat Toomey.

Here are five key factors in the contest, as described by operatives, insiders and analysts from both parties in recent weeks – including one wild card in the race and why the Democratic primary might not actually be the most important one to watch.

1. Party Support
Start with the most obvious factor: national Democrats and many insiders in Pennsylvania are wary of Sestak, his unorthodox approach and his free-swinging persona. It's a safe bet that many of the party's big names will support McGinty. (She landed union support Tuesday, announcing on Twitter that the United Steelworkers had endorsed her.)

But does party backing really matter?

One of the big reasons some Democrats don't like Sestak is that he defied them by running in 2010 even after the late Sen. Arlen Specter switched parties. But that race shows that party support isn't necessarily a predictor of success in Pennsylvania – everyone from President Obama on down was on Specter's side. Sestak still won and came within two percentage points of Toomey despite running in a strong Republican year.

Sestak is already playing the outsider card to his advantage: "too much is at stake for another six years of an establishment politician," he said in a release Tuesday. It's a stance that might have appeal at a time of massive frustration with typical politicians. And it will allow Sestak, despite two terms in Congress, to paint McGinty as part of the political elite.

There's one key difference this time, though: when he ran against Specter, Sestak had to convince the Democratic base to keep voting against a guy they had already been voting against for decades. Against McGinty, he faces a woman with deep ties to the party, including the Clinton, Rendell and Wolf administrations.

2. The Money Race
On the other hand, support from the party apparatus could be huge in the race for campaign funds. Sestak had $2.2 million on hand as of June 30, and McGinty didn't put up big numbers in her gubernatorial campaign last year, when she was free from the tighter caps on federal donations.

But if McGinty can tap into the kind of networks built by Rendell and wins support from EMILY's List (which has encouraged her to run) and the Democratic Senatorial Campaign Committee, those official channels can help her make up financial ground, while also providing the kind of professional support that can help in a nationally-watched race (and which Sestak largely spurns in favor of people he knows well).

The DSCC, of course, says it is staying neutral. It will at least want to see that McGinty can mount a credible run. But as long as she shows reasonable progress, most people expect her to have the national party's backing.

3. Women
McGinty didn't directly hit this point in her campaign roll out. She probably didn't have to – it's obvious enough. The idea of a Hillary Clinton-McGinty ticket, one trying to make history in the White House, one trying to make Pennsylvania history in Congress, is sure to thrill plenty of Democrats.

Pennsylvania has never had a woman governor or senator and has no women in its 20-person Congressional delegation. It's easy to see the appeal on the left of breaking that barrier.

On the flip side, the fact that those state-level barriers still stand is a reminder that Pennsylvania is not always a friendly place for women candidates. McGinty will also have to show that she's a competent, credible option.

But should she win the primary, having a woman opposing Toomey would provide a different contrast than a Sestak-Toomey rematch.

4. Foreign Affairs
Few elections hinge on foreign affairs, but Franklin and Marshall pollster Terry Madonna said this year might be an exception: "I think it's a wild card."

President Obama's nuclear deal with Iran is dominating the news right now and likely will continue to do so this fall as Congress prepares to vote on the pact. Recent months have also seen Russia, China, ISIS and Cuba grab national attention.

Seeming to sense the importance of the issue, Toomey, mainly known for his fiscal bonafides, has been flexing his muscles on Iran since the start of the year, calling for tougher sanctions and sharply criticizing the nuclear deal. After months of ignoring Sestak, Toomey last week made Iran the first issue that he engaged on head-to-head.

This is Sestak's home turf: he is a former three-star Admiral who commanded an aircraft carrier battle group in Afghanistan and Iraq. He was director of defense policy on President Bill Clinton's National Security Council.

McGinty's work has mostly been on the environment – certainly a big topic, but not one dominating the news right now like foreign affairs. Could voters worried about the U.S. approach to the world turn to a military man?

5. The GOP Primary
While we all focus on the Sestak-McGinty rumble, longtime Democratic analyst Larry Ceisler pointed out that the primary that might matter most is the one for the Republican presidential nomination.

When there's an incumbent on the ballot, elections tend to be about the incumbent, he reasoned, and Toomey doesn't evoke the kind of strong feelings that have voters either clamoring for more or storming his office with pitchforks.

Put him on the ballot with a moderate Republican who can run well in Pennsylvania, and Toomey will have a shot to win over enough swing voters to stay in office, even if the commonwealth follows recent history and again goes blue in the presidential race. But if one of the more conservative Republican contenders leads the GOP ticket, and Toomey has to answer for their policies or Democrats romp in the presidential contest, the gap could be too much to make up.

Democrats are eager to take on Toomey because he won by just 2 percentage points in 2010 and now has to run in the kind of year that typically brings lots of Pennsylvania Democrats to the polls. Republicans counter that Specter (in 2004) and Rick Santorum (in 2000) held onto their Senate seats despite Democrats winning the presidential contest.

In those races the presidential races stayed close: Al Gore won Pennsylvania by 5 percentage points in 2000 and John Kerry took it by about 3 in 2004. Obama beat Mitt Romney by about 5 in 2012.

Stay within that range, and Toomey has a good shot. A bigger gap, and his task gets much harder, regardless of which Democrat he faces.

You can follow Tamari on Twitter or email him at jtamari@phillynews.com.