President Barack Obama has opened his final presidential campaign — against Donald Trump.

Though his name won't be on November's ballot, Obama is slowly embracing his role as the anti-Trump, using the contrast between himself and the boastful billionaire to paint Trump as anything but presidential.

A Trump victory in the presidential race would mark an overwhelming rebuke to Obama and the likely demise of many of his policies. So with Hillary Clinton and Bernie Sanders still fighting it out in the Democratic primary, it's fallen increasingly to Obama to take on Trump in ways that no other Democrat can.

For months, Obama and his aides mostly avoided getting dragged into the fray or letting the campaign din distract from Obama's agenda. The White House would sidestep questions about the latest Trump controversies, refusing to turn Obama into a pundit on the race to replace him.

When Obama waded in, it was only to offer implicit rebukes of the Trump phenomenon, such as Obama's assertion in September that "America is great right now" — a not-so-veiled reference to the business mogul's campaign promise to "make America great again."

Now the Trump critique is coming with increasing frequency and ease. Asked Tuesday whether Trump's proposals were already damaging US relations, Obama answered unequivocally: "Yes."

"I am getting questions constantly from foreign leaders about some of the wackier suggestions that are being made," Obama said. "They don't expect half-baked notions coming out of the White House. We can't afford that."

The Democratic National Committee quickly circulated video of Obama's remarks, arguing they illustrated how Trump "simply doesn't have the temperament necessary to be commander in chief."

Yet by calling attention to Trump's positions, the White House risks further elevating him, while giving Obama's critics a fresh reason to get behind the billionaire businessman.

Obama has said repeatedly he doesn't believe Trump will win, and White House officials said there was no concerted effort to insert Obama more visibly into the election debate. After all, every minute Obama spends talking about Trump is a minute wasted when it comes to Obama's many unfinished pieces of business.

Still, the president in recent days has rarely passed up a chance for a Trump takedown — especially on foreign policy, where Obama's status as commander in chief gives him a unique perch to besmirch Trump's approach. Closing out a nuclear security summit last week, Obama said Trump's suggestion that South Korea and Japan develop nuclear weapons "tell us that the person who made the statements doesn't know much about foreign policy, or nuclear policy, or the Korean Peninsula."

Republican US presidential candidate Donald Trump speaks at a town hall event in Appleton, Wisconsin, March 30, 2016. Photo: Reuters

"Or the world generally," Obama added wryly.

And in his latest rebuke, Obama unloaded on Trump's proposal to compel Mexico to pay for a border wall by threatening to cut off remittances that Mexican immigrants in the US send back home. Asked about that idea, Obama issued a point-by-point rebuttal, arguing that would actually increase the flow of immigrants into the US and that tracking huge numbers of remittances was impossible.

"Good luck with that," Obama said.

Obama's public scolding of Trump, who for years peddled inaccurate claims about Obama's birth certificate, dates back to 2011, when Obama roasted him at the glitzy White House Correspondents' Association Dinner. Trump was visibly humiliated as Obama lobbed joke after joke at him on national television.

Obama, echoing the broader message from Democrats this year, has stressed that Trump isn't the only Republican espousing "draconian" rhetoric about Mexicans, Muslims and others. Yet the brunt of Obama's criticism has centered on Trump, who has the best mathematical path to the Republican nomination despite losing to Texas Sen. Ted Cruz in Wisconsin on Tuesday.

Though Clinton and Sanders have both vigorously attacked Trump, neither has been able to focus exclusively on the Republican as their battle for the Democratic nomination continues. Obama hasn't endorsed either of the Democrats or campaigned on their behalf, leaving his condemnation of Trump as his primary foray into the race to date.

The White House said once the Democrats choose their nominee, Obama will be out in full force campaigning, raising money and activating his own supporters. White House spokeswoman Jennifer Friedman said Obama would cast the general election as a choice between building on Obama's policies and abruptly reversing course.

"This is a choice that the president does not take lightly, and is something he will lay out for the American people with increased frequency in the weeks and months ahead," Friedman said.