Democrats are a considerable underdog to take control of the senate. It is actually less likely that Democrats take the House than Republicans continue to control the senate. The betting odds on Paddypower are Republicans 2/5 or -250 to retain the senate, while Democrats are 4/6 or -150 favorite to retake the House:
https://www.paddypower.com/politics/usa-congressional-electionsIt is all about numbers and terrain. These senate seats were won in the Obama re-election of 2012. Many are in red states that Democrats are defending. Republicans aren't defending much of anything in unfriendly territory, and they really aren't defending much of anything period. Democrats are on defense in the senate. They could actually lose seats, if all the narrow results broke away from them.
Controlling 67 seats is impossible. There is greater likelihood that the world record in the mile run is lower than the world record in the 100m next year, than the odds of Democrats owning 67 seats. That may be hyperbole but barely. Zero chance of either.
House control will come down to independents and how they view Trump. They pushed him over the electoral line in 2016 but that block has shifted against him, although not as lopsided against him as November 2017. Make no mistake, the euphoria regarding a massive blue wave began in late 2017 but Trump's approval rating has slowly upticked since then, and that impacts the bottom line. Many Democrats are overconfident right now, expecting 30+ seats. Meanwhile the GOP and surrogates are going to dump tens of millions into those races. I would be thrilled with Democrats barely getting over the magic number. The problem is Democrats due to gerrymandering basically need a 7-8 point generic House edge to yank those 23 or 24 seats. Tall order.