It's possible. There's a lot of republicans who are more centrist saying they'll vote Clinton if he runs and another group that just won't vote. If trump running suppresses the GOP vote but bolsters the DEM vote then it's possible. Though probably still really difficult. If trump runs 3rd party there's an argument to be made that this will actually increase GOP turnout and likely be bad for DEMS down ticket even though it would ensure a Hillary win.
If you assume most people who come out to vote will vote down the line - ie if you vote a republican president, you will vote republican everything else, and likewise democratic president then democratic everything else - its still really to early to figure out what Trump vs Hillary would be like.
I could counter that just as many people who decide to stay home if Trump is on the ticket, there would be just as many democrats who are un-enthusiastic about Hillary and stay home. Historically older, white voters tend to be pretty reliable voters and its harder to get minorities and young people to get out and actually vote. Obama definitely energized the young into coming out. I think Hillary will definitely energize many women to come out, but will she get young people to come out? End of the day you are going to go out and spend an hour or two of your time because you believe in something, not to vote against someone else.
Historically as well voters tend to split control of President/Congress, and stick with incumbents. The majority of seats up for grabs will be held by incumbents. Democrats would need to pick up 30 seats to gain control in the House. In 2012 Obama won 209 congressional districts, the democrats currently control 188 of those (they lost 21 so far). So they'd have to win all those back plus 9 more districts that have traditionally voted Republican. Not impossible, but fairly difficult.
Only twice have more than 30 House seats switched hands during a presidential election - Ronald Reagan in 1980 and LJB in 1964 (a year after the Kennedy assassination).
I don't really have a problem with Obama passing this executive order, almost certainly it'll get shot down in courts and even if it doesn't, it doesn't really change much. 82% of guns used in mass shootings were purchased legally by the owner and passed a background check. The gun show loophole is real for sure, but people will just agree to meet up afterwards to conduct the sale so not really sure what its going to change. Online dealers can't directly ship guns to people, they must send them to a local FFL dealer who then runs a background check. But the whole thing is going to rile up a lot of people, not sure if it helps or hurts Hillary but it definitely helps Trump and the Republicans.
