Obama's approval rating is at its highest point in years, and it could be a huge boon for Hillary Clinton

Barack Obama is prepared to campaign for his party's likely nominee more than any sitting president in recent history - starting next week in Green Bay, Wisconsin.

That could be a big problem for the GOP. And a huge boon for Hillary Clinton, the presumptive Democratic nominee whom Obama finally formally endorsed earlier this week.

"I'm fired up," Obama said in a video endorsing Clinton.

And ready to go. Last month, an NBC/Wall Street Journal poll found that President Barack Obama's approval rating had jumped to 51% - its highest point since his second inauguration.

NBC's team of political analysts called it the "most important number" out of the new poll.

"Why is it important? Because it means that Obama will be an asset to Hillary Clinton on the campaign trail unlike he was in the 2014 midterms, when his approval rating was in the low 40s," NBC's Chuck Todd, Mark Murray, and Carrie Dann wrote.

The 50% level might seem arbitrary. But historical precedent suggests it could bode well for Clinton, Obama's former secretary of state.

Early this year, Obama's approval rating hit 50% in the weekly average from Gallup's daily survey. Almost three months later, his number in Gallup's poll stands at 54%, his highest level since January 2013. For Obama, whose approval ratings have been stuck in the mid- to low-40% range for much of his second term, it was a notable bump.

"While it's hard to pinpoint precisely why Obama's approval rating has risen among Democrats recently, there are a number of plausible explanations," wrote Andrew Dugan, a Gallup analyst, and Frank Newport, the organization's editor-in-chief, in a post earlier this year.

One of the explanations, the pair concurred, was that "the unusual status of the Republican primary race - exemplified in particular by frontrunner Donald Trump's campaign style and rhetoric - may serve to make Obama look statesmanlike in comparison."

Trump has come into Obama's crosshairs recently. And with good reason: More so than at any other presidential hand-off in recent history, so many elements of the current administration's legacy are at stake.

The presumptive Republican nominee has pledged to undo signature achievements on healthcare (the Affordable Care), the environment (historic new regulations aimed at curbing climate change), and foreign policy (the Iran nuclear deal).

Ben LaBolt, a former spokesman for Obama's presidential campaigns, told Business Insider that those themes will become evident as the president launches into what will be his final campaign: preventing a Trump presidency. And LaBolt suggested Obama is the perfect messenger.

"President Obama gives Hillary Clinton a hat trick: He can help unite the party by bringing out Bernie Sanders supporters into her camp, deliver an aggressive contrast about the threat posed by Donald Trump, and ensure that all the supporters of the Obama coalition show up in November," LaBolt said.

He added: "Not only does he have strong standing among Democrats and independents, but he has a unique ability to mobilize the young voters and diverse communities she'll need to win."

Obama's approval ratings at this point are far better than his predecessor, President George W. Bush, off whose unpopularity Obama thrived during his 2008 run. His level is most directly comparable to former President Ronald Reagan, who in March 1988 held a 51% approval rating, according to Gallup.

That same year, voters selected George H.W. Bush - Reagan's vice president - to succeed him.

"Yes," said Ari Fleischer, President George W. Bush's press secretary, when asked earlier this year if Obama's apparent rising popularity poses a problem for the Republican Party.

"Certainly, going into an election spring and summer, it's better to have an incumbent president increasingly popular rather than less popular if you're the incumbent party," he told Business Insider.

The numbers present a striking contrast to some data points associated with the current Republican presidential frontrunner.

Trump's favorability numbers are without precedent for a modern general-election candidate, according to the NBC/Wall Street Journal poll. Overall, 58% of voters viewed him unfavorably, according to the survey. And his net-favorability rating was negative 29 - only slightly better than two months before, when he garnered the worst net-favorability rating in the history of the NBC/WSJ poll for a major presidential candidate.

"I've been doing this [since] 1964, which is the Goldwater years," NBC/WSJ co-pollster Peter Hart told NBC of the relative unpopularity of many of the candidates earlier in the year. "To me, this is the low point. I've seen the disgust and the polarization. Never, never seen anything like this. They're not going up; they're going down."

However, Obama's approval rating at this point is actually lower than that of President Bill Clinton in March 2000. But, as Gallup has noted, the 2016 election is somewhat different than the one that featured George W. Bush running against Al Gore, Clinton's vice president.

Hillary Clinton is attaching herself to much of Obama's legacy. And Obama remains favorable to wide swaths of constituencies whom Clinton needs to turn out to vote in November. The president held high approval ratings among African-Americans (90%), Democrats (82%), Latinos (73%), and voters aged 18 to 34 (64%).

And despite the strong primary challenge from Sen. Bernie Sanders, in many ways, Clinton has run an incumbent-style campaign, and she has much of the party's establishment rallying behind her candidacy. That wasn't the case for Gore in 2000.

As Gallup's Dugan and Newport wrote earlier this year:

In comparison, the two most recent candidates running to succeed a two-term president of the same party - John McCain running to follow the unpopular Bush, and Al Gore trying to succeed the popular but scandal-prone Bill Clinton - went to greater pains to ensure they were not associated with the outgoing president.

They concluded: "Prior to that, George H.W. Bush in 1988 presented himself as a natural heir to the Reagan legacy and was able to win his own term."