Republicans are legitimately starting to panic over the midterms. It's gotten so bad that the cracks are starting to show as the party is on the brink of writing off the House as a lost cause:
Republicans are increasingly worried they will lose control of the House in the midterm elections, furiously directing money and resources to hold and potentially boost their narrow majority in the Senate.
Without the Senate, Republicans will no longer be able to pack the courts with far right extremist judges. Worse, should a Justice of the Supreme Court retire or die, it doesn't take a genius to see that Democrats will happily follow in Mitch McConnell's footsteps and keep the seat open indefinitely.
But that is not the most frightening thing for Republicans:
...some in the GOP fear, Democrats could use complete control of Congress to co-opt the ideologically malleable president and advance their own priorities.
Trump is not so much ideologically malleable as devoid of ideology. If Democrats put bills in front of him with a lot of public support, Trump will want to sign them and tear into Republicans if they try to block them. Trump is a "Republican" in the sense that he's a racist and racists tend to vote Republican. Economically, he has about as much loyalty to the Republican donor class and their needs as he does to his wives and his wedding vows. As long as what the Democrats ask him to do will be popular with the public, he'll agree because the only thing Trump cares about is being popular.
The danger here for Republicans is that a lot of the things Democrats want are actually very popular with the public, including Republican voters and Trump really likes to sell himself as an economic populist. If Trump were to go on TV tomorrow and say America is going to raise taxes on the rich to make Social Security safe for the next 100 years, Fox News and Republicans in congress would foam at the mouth but Trump voters wouldn't bat an eye. The same is true of stabilizing healthcare costs, increasing the minimum wage, passing paid maternity leave, etc. Trump has been attacking those things because Republicans tell him to and they are not popular moves. Should Democrats get him to do anything and Trump sees a sudden surge in popularity, "Chuck and Nancy" are going to be his new best friends and Republicans can go scratch.
It may be distasteful working with a monster like Trump but since Republicans will never vote to remove him from office, Democrats can either grind Washington to a halt for 2 years which helps no one but Republicans, or they can spend the time and effort to mold the Play-Doh President into something useful and get at least a few things done for the people until it's time to vote him out of office.