Too late actually to compete in the 2012 Republican Presidential primaries, Donald Trump prepared in August of that year a TV ad to show how he would have been campaigning against Obama’s re-election if he were to have been running against him.
Here it is:
Because the ad features (at 2:37) this headline, «U.S. Unemployment Climbs to 8.3%», from the Chicago Tribune on 3 August 2012, it had to have been prepared after that date.
At 2:08 in this video appears Obama’s 26 March 2012 off-mic private assurance to Putin through Medvedev, that when Obama gets re-elected he’ll end America’s continuing Cold War against Russia. (Obama was specifically talking there about his willingness to accommodate Putin’s objections against installation in Europe of America’s anti-ballistic-missile (ABM, or also called Ballistic Missile Defense, BMD) system — which promise to Medvedev turned out to have been a lie. (And Obama was, even then,already planning his coup to overthrow the Moscow-friendly leader of Russia’s neighboring Ukraine, Viktor Yanukovych, which coup culminated in February 2014.) Trump’s comment there to Obama about that off-mic clip was: «Frankly, it makes us worry whether we can trust you at all». However, this was actually a cheap shot by Trump, because everyone knows that negotiations between countries take place in secret, just as any negotiations of any deal necessarily must. If and when a deal is finalized, it normally (but not always) is made public; and, if the deal is between democratic nations, then the deal doesn’t go into effect unless and until it becomes ratified by the legislatures of the respective countries. But Trump wasn’t criticizing Obama for lying to Putin; he was criticizing Obama for promising there to negotiate with Russia (something that Trump himself as a 2016 Presidential candidate very publicly promised to do). Trump in 2016 publicly promised to do, what Obama in 2012 privately promised to do and was in 2012 criticized in this ad by Trump for having done.
Trump’s ad closed (3:40): «President Obama, you’re fired».
The Republican National Convention occurred during 27-30 August 2012, and so this Trump TV commercial was probably created in order for him to be able to show it to potential financial backers for a 2016 White House campaign by Trump — the type of ads he would be running against, presumably, Hillary Clinton.
As regards why Trump didn’t run against Obama in 2012, one can only speculate.
Trump knew, at least as early as the 30 April 2011 White House Correspondents Dinner, where Obama referred contemptuously to Trump when addressing Washington’s elite, that Obama was an extremely formidable political opponent, who possessed an unmatched ease and grace, even when revenge-seeking. Some commentators say that Trump entered the 2016 Presidential contest in order to exact revenge against Obama by destroying Obama's Presidential legacy, but there’s no significant Obama legacy to destroy (other than Obamacare, which is already failing of its own accord); and, anyway, the hypothesis that Trump sought the Presidency for reasons having to do with that skewering of him by Obama is very weak and is basically unsupported.
Furthermore, there is solid evidence that Obama not only wanted Hillary Clinton to beat Bernie Sanders to become his successor, but that Obama absolutely loathed Trump and wanted Hillary’s campaign to clobber him and remove him from America’s political future: During the White House Correspondents Dinner on 30 April 2016, Obama skewered Trump yet again, at 9:40 and 14:00 and 20:00 in the video of the speech, and Trump absented himself from that occasion, so as to leave Obama punching an absent bag. In that same speech, Obama praised (17:00) «Hillary’s toughness, her smarts, her policy chops, her experience,» and mentioned Bernie Sanders (14:55) as if he were just the Democratic Party’s cuddly pet and «especially his appeal to young people» (the main people petting him) — as if Hillary were the candidate preferred by the adults (who knew better).
Trump knew better than to pit himself politically in a contest against the most gifted American politician since — well, perhaps ever. McCain and Romney had been Obama’s punching-bags, and cleared that competition away from the Republican primaries field now in 2016 — time for Trump to step into the ring, finally, against the glass-jawed Hillary Clinton, and deliver the knockout blow that, perhaps, only Stephen Bannon was expecting.