No. There are currently only 7 districts that'll be competitive where a Republican is retiring. Democrats need to win 30 seats to get a a majority. Even if Democrats won every competitive district they'd still fall several seats short. And it's unlikely they'll win every competitive seat. Even with a wave. Which likely won't happen. History shows that a Presidential wave for an open seat happens for the party that doesn't have the White House, not the one that does.
What if the Republican nominee is Ted Cruz or Donald Trump? Surely then?
Not even then. It's far from certain that either Cruz or Trump would lose in a landslide. Most polls show Trump and Cruz only doing a few points worse than Marco Rubio in a general election. The RCP averages have Rubio winning by 2.5, Cruz winning by 1.3, and Trump losing by 2.7. But even if the Republican nominee does lose in a landslide there are two things which tell us that wouldn't doom Republican congressional candidates.
When Nixon and Reagan won big landslides in 1972 and 1984 the Democrats actually did better congressionally than they did four years earlier. People didn't vote congressionally as a reaction to fringe Democratic candidates George McGovern and Walter Mondale. They did vote big for Democrats in 2008 but that was a reaction to the Republican party not the Republican nominee.
The second thing is the type of candidates Cruz and Trump are. They have strong appeal to people on the fringe of the party, those that might be less active voters and might stay home if they weren't happy. So those people will vote. Establishment Republicans would be the ones that people believe would stay home, but they rarely stay home because they're loyal to the Republican party. Maybe they could be enticed to vote for Hillary Clinton but they're not going to pass up an opportunity to vote for an establishment Republican for congress.
Okay, okay. I get it. The Republicans keep the House. I suppose they keep the Senate too.
Not necessarily. Because the Senate has fewer races, 34 instead of 435, it's far more dependent on the circumstances of those races than the environment. With 435 races, the candidate quality tends to even out, but with the small number of senate contest that isn't necessarily the case. Democrats won five states Romney won in 2012. That alone should tell us that a big Clinton victory in a state doesn't mean Democrats will win the Senate seat. Here Democrats need to win 6 seats among 10 competitive. That's possible mathematically the way winning 30 in 28 competitive districts isn't. Democrats already look strong in Republican held Wisconsin and Illinois. If they can retain Colorado and Nevada they'd need only win two from Florida, New Hampshire, Ohio, Pennsylvania, North Carolina, and Arizona to get to 50-50. Control of the Senate is very much in play.