Neither Romney nor Obama seem likely to have sex scandals. Other sorts of scandals, like Romney illegally evading taxes or Obama engaging in corruption, would be more likely.
No Congress will be workable with Ron Paul. The Republican leadership would know his full plan would be disaster, and even most of the right of the Party would never cut the military like he wants.
If Ron Paul won the Republican nomination, his anti-war and anti-Israel positions would result in the interventionist establishment abandoning him. They wouldn't just want him to lose, they'd want him to be destroyed and humiliated. Sheldon Adelson and his billions would back Obama to ensure that Ron Paul lost as badly as possible.
Ron Paul would be the Republican version of McGovern. The Southern Democrats and Union Democrats hated Nixon, but they hated McGovern more so they defected. The same thing would go on here.
Ron Paul's performance's in the debates were overstated; I easily saw through his anti-government platitudes throughout. Votes from the youth aren't going to come once Obama, one of the greatest campaigners of modern times, hammers Paul's homophobic, anti-choice (as in he agreed with Todd Akin!), anti-evolution, record. He'd steal a few votes away from the Constitution Party from isolationist conservatives, and he'd steal a few votes away from the Libertarians from conservatives who like marijuana, but those would just be marginal gains and no where near enough to compensate for the complete abandonment of the Republican establishment.
Moderates would sweep against Ron Paul, the politician with the most right-wing economic record of anyone who's served in Congress in the last century. Big business is not going to back "No bail outs" Paul. The Social Conservatives are going to be unenthusiastic backing someone against the drug war, even if he is otherwise perfectly reactionary for him. The hawks would throw all their support and money behind Obama to stop Paul. The unions are going to fight "National Right to Work" Paul. Women aren't going to break for Ron "Honest Rape" Paul and his platform to ban abortion via constitutional amendment. The racial minorities aren't going to back someone against the 1964 voting rights act who wants to abolish all forms of welfare and increase border security. LGBT people are going to stand against someone who backed DOMA, Proposition 8, opposed ENDA, and voted against hate crimes legislation. No one who backed Obama in OTL 2012 is going to defect, but a lot of people who backed Romney will either defect to Obama, vote third party, or stay home.
Ron Paul's going down like McGovern in 1972 unless it turns out that Obama was buying underage sex slaves from the Taliban and Mexican Drug Cartels.