50/50 chance the Democrats lose the Senate. Obamacare likely gets repealed with no replacement, as repeal AND replace didn't become a thing until Obama secured a 2nd term with Obamacare on the ballot. A Tax Cut for the wealthy as large or larger than Trump's is passed before November of 2014 and Scalia and Kennedy are replaced before then as well. A neoconservative foreign policy makes a comeback with Romney to. After that, Romney rides a recovering economy to a second term in 2016 and replaces RBG this year, though Romney replaces her with someone more qualified and less controversial than ACB.
An 8 year Romney Presidency, which is a less "compassionate" version of George W. Bush with dashes of Trump's immigration and China polices, ends with millions of people not having health insurance, a 6-3 Conservative Supreme Court, a war in Iran and/or Syria, a recession and a pandemic (though one he handles better than Trump).
I have a hard time seeing them hold the senate with the six year itch. It hasn't happened since 1966.