One early takeaway of the GOP's invisible primary? A whole lot of people sure look like they're going to run.

On Friday, Mitt Romney told a group of donors that he was considering another run for president. Minutes later, a new report said Marco Rubio had gotten approval from his family for a run, and that he wouldn't be scared away by Jeb Bush's fundraising.

Earlier in the week, Mike Huckabee ended his Fox News show to explore a bid, Jeb Bush launched a fundraising operation, Scott Walker's hiring of a likely campaign manager became known, and Chris Christie reportedly decided to move up his timetable for an announcement. Rick Santorum and Rick Perry also said they were seriously considering running. Less plausible candidates like Ben Carson, George Pataki, and Carly Fiorina have previously signaled their interest.

Now, it's not certain that all of these people will end up running. And many of those who do could drop out well before the voting begins — there tends to be a winnowing of the field as some candidates fail to raise enough money and win support.

But unlike in 2011-2012 — when many potentially strong contenders shied away from a bid — Republican hopefuls seem to be much more eager to run this time around. Here are five reasons why.

1) Jeb Bush would make a weak frontrunner

For most of 2014, there wasn't really anyone who could be deemed a frontrunner in the GOP field. With Christie sidelined by Bridgegate , Rand Paul still getting a mixed reception from the GOP establishment, Scott Walker busy with his own reelection, and Marco Rubio damaged by his support of the Senate immigration bill, there seemed to be a vacuum in the field.

Enter Jeb Bush. With an unexpectedly early announcement of an exploratory committee and an aggressive fundraising push that Michael Bender and Jonathan Allen of Bloomberg Politics describe as "shock-and-awe," he's inarguably made a strong debut and forced the other potential candidates to move up their timetables for running.

Yet Bush has been out of politics for eight years. His positions on immigration and education aren't popular with conservatives. And his last name could prove to be more toxic than GOP elites currently expect — in general election polling, or even in the primary. As Ben Smith argued, he might not be in touch with how the GOP has changed since the rise of the Tea Party — and might stumble on the trail. Bush has even said that a GOP candidate this year should be willing to "lose the primary to win the general" — and maybe he will! (The first part, that is.)

2) The eight-year itch

Since Harry Truman's presidency ended, there's only been one time where a party has held onto the White House for over eight years — the Reagan-Bush reign of 1981-1992. Democrats and their likely nominee Hillary Clinton would be attempting to match that rare streak.

But as Brendan Nyhan wrote at the Upshot, voters seem to get tired after eight or more years of the same party, in what political scientist Alan Abramowitz calls the "time for a change" effect. More and more voters start to think that the opposition should get a shot.

We've gotten some good economic news recently, and if that continues, any GOP candidate will face a tough time winning in 2016. But the economy that year will matter much more than what's happening now, and we simply don't know what conditions will be yet. And, of course, Jeb's older brother won the presidency over Al Gore when the economy was quite strong, and when President Bill Clinton was much more popular than Obama is now.

3) The 2010 and 2014 elections helped expand the GOP field and embolden the party

The Obama midterm years have just been amazing for the growth of the Republican Party, so there are simply more credible contenders to go around. In 2010, the party elected a host of new governors, senators, and members of Congress including Rand Paul, Marco Rubio, and Scott Walker, as well as other potential candidates like John Kasich. (All but one GOP governor running again in 2014 won.)

Beyond that, the party was emboldened by its sweeping 2014 wins. With the takeover of the Senate and most state governments, conservatism seems to be on the ascendancy, and Democrats in decline. And the news has been dominated by crises all over the world, further feeding the sense that Obama has failed. Obviously, the 2010 GOP victories didn't lead to Obama's defeat in 2012. But many potential candidates feel like they've learned from Romney's mistakes — including Romney.

4) Non-serious candidates can become more famous by running

Some people run for president because they think they can win. Others run because it's an easy way to get the press — and your party's base voters — to pay attention to you. This is particularly true in our modern era of many televised debates and intense media coverage, and in 2011-2012 unlikely candidates like Herman Cain, Newt Gingrich, Michele Bachmann, and Rick Santorum all had their moments in the sun. A no-hope candidate can use this brief spotlight to push some preferred policy ideas, as Ron Paul did. He or she can also have more mercenary motivations — for instance, hoping that newfound fame will lead to a lucrative book deal or media gig.

5) They can all position themselves as an alternative to Hillary Clinton

The funny thing about Hillary Clinton being the overwhelming favorite for the Democratic nomination is that practically any Republican can contrast himself or herself to her.

If you're a young and new candidate, you can point to Obama's defeat of Clinton in 2008, and argue you could do the same as the candidate of change. Why would the GOP choose a candidate of the past when they could be the party of the future?

Alternatively, if you're an older, better-known candidate, you could make the case that Clinton is a formidable opponent, and that the GOP needs someone experienced and tested to take her on. If you're from a political dynasty, you can even argue that that won't hurt too much in the general election, since Clinton is too (sort of).

In the view of many, Clinton's past year showed off many of her weaknesses, and made clear that she's vulnerable. We'll see how that plays out — but for now, it's an argument for any Republican to jump in the race.