Is this Hillary Clinton's most important friend?

Obama Hillary Clinton endorsement_00000000
Obama Hillary Clinton endorsement_00000000

    JUST WATCHED

    Obama endorses Clinton, calls for party unity

MUST WATCH

Obama endorses Clinton, calls for party unity 02:13

Story highlights

  • Errol Louis: President Obama has strong approval numbers and appeal in key swing states
  • A popular president can help his party keep the White House, Louis says

Errol Louis is the host of "Inside City Hall," a nightly political show on NY1, a New York all-news channel. The opinions expressed in this commentary are solely those of the author.

(CNN)President Barack Obama ended months of waiting and endorsed former Secretary of State Hillary Clinton to succeed him. Naturally, Donald Trump took to Twitter.

"Obama just endorsed Crooked Hillary," Trump tweeted shortly after the endorsement. "He wants four more years of Obama -- but nobody else does!"
    That last part is simply not true. Quite a few people like the President and his policies in places and for reasons that should have Team Trump worried.
    Errol Louis
    For starters, Obama's endorsement carries the drama of being history in the making: the first black president backing the first female major party nominee for the White House.
    Beyond the symbolism, landing Obama's backing strengthens Clinton's link to an administration that's ending with strong public approval numbers and a powerful appeal in key swing states.
    When Obama hits the campaign trail for Clinton this summer, he'll take high job approval ratings with him. An average 49% of voters have been satisfied with Obama over the last month, which is roughly 6 percentage points higher than when the year began.
    That matters. Recent history shows that with few exceptions since World War II, a popular president (as measured by the polls) has helped his party hold the White House after his time is up.
    When then-Vice President George H.W. Bush ran to succeed President Ronald Reagan in 1988, the Gipper had a 60% approval rating, which helped Bush win the White House.
    President Bill Clinton had an even higher job approval of 66% during the last weeks of his presidency in December 2000, but his would-be successor, Al Gore, unwisely chose to distance himself from the scandal-laden Clinton administration. Gore lost.
    And just as a popular president can help his party hold the White House, an unpopular one can hurt his party's chances. Back in 1952, Democratic President Harry Truman neared the end of his presidency with a dismal 22% approval rating. Republican Dwight Eisenhower easily won the presidency against Democrat Adlai Stevenson.
    More recently, George W. Bush's second term was marred by tremendous unpopularity; at the close of the 2008 race for president, the departing president had an anemic 25% approval rating, according to Gallup, and left the White House two months later with a 22% approval rating and a whopping 73% disapproval rating in a New York Times poll.
    Republican nominee Sen. John McCain did his best to distance himself from Bush. McCain didn't campaign with the sitting president, but Bush's unpopularity helped doom his chances, and Democrat Obama won.
    Fast-forward to this summer, when a popular president will hit the campaign trail. Almost as important as Obama's overall popularity is his connection to key constituencies in battleground states.
    The President's arm around Clinton will become particularly crucial for Latino voters, who have been some of his biggest supporters when it comes to the Affordable Care Act, otherwise known as Obamacare. The Washington Post surveyed Hispanic voters in February on a slew of desires out of the nation's next president; 54% of those polled said they would want the next commander in chief to keep the signature health care legislation.
    That support will be important in swing states such as Colorado, New Mexico and Florida. Hispanics made up 17% of voters in Florida four years ago, up from 14% in 2008, Politico recently found. The demographic also grew by 1 percentage point in Colorado between 2008 and 2012.
    Obama will also help energize and turn out black voters in states such as Ohio, Virginia and North Carolina, where Clinton holds marginal leads. One in five voters in Virginia and North Carolina is African-American, according to Politico, a key bloc that has helped turn the state blue in recent elections.
    A final boost from Obama adds another high-level surrogate to Clinton's team on the campaign trail, along with Vice President Joe Biden and ex-President Bill Clinton. By contrast, Trump has alienated the last two standard bearers of his party.
    Meanwhile, the 2012 GOP nominee, Mitt Romney came out in March and trashed Trump's policies, calling the real estate tycoon "a phony, a fraud." And it's no surprise that McCain, the 2008 nominee, and Trump haven't seen eye to eye since the candidate insulted the senator's war record and the fact that he was a prisoner of war during the Vietnam War.
    So Obama's long-expected endorsement of Clinton is anything but routine. It's a major coup that helps her immeasurably -- in ways Trump can only dream about.