If he's elected, one presumes that the Republicans would remain in control of the House (at least - we have a big majority there). Likely, Republicans will also maintain Senate control. Heck, their majorities will probably expand. So how, exactly, is an impeachment going to happen then?
As for Dem rule for good, I'd be a WHOLE LOT more concerned about a much more realistic possibility, one which has been planned for about 50 years, which is a massive change in the electorate brought about via immigration. Get amnesty for 30 million or more illegals, and tens of millions of new Dem voters come into our political process. Where's the Constitution going to be then, when they can shut us out of the White House for 20 or more years and elect another Obama-type who will run roughshod over it? You want to preserve the Constitution? Yes? Good, so do I! Let's stop amnesty, let's get rid of as many illegals as possible, let's stop the birthright citizenship BS, let's end anchor babies...in short, let's try to reverse what the anti-Constitutional Dems have foisted on us since the 1965 Immigration Act. Do those things, and the electorate will be a good deal more conservative in 5 or 10 years - and then the Constitution will be a lot safer.
“The perfect is the enemy of the good.” General George S. Patton, Jr.
“Freedom is never more than one generation away from extinction. We didn’t pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children’s children what it was once like in the United States where men were free.” - Ronald Reagan
Just my $0.02.