Now, if a sitting president of the United States stood in the middle of Fifth Avenue, shot a random person in broad daylight, and was caught with a smoking gun, it’s probably a safe bet that Congress would vote to impeach him, convict him, and remove him from office, despite their partisanship. (Probably.)
But most political scandals are not so indisputable, damning, or well-documented as that. And on any matter where there is some sort of plausible deniability for the president, his political allies will have very strong incentive to give him the benefit of the doubt, even if it means twisting themselves into knots.
So as long as Republicans control Congress, there will be a very high bar indeed for any effort to impeach Trump to gain traction — some incredibly damning evidence of some indisputably serious crime would likely have to emerge. And even if Democrats retake the House, they’d still need to come up with a case strong enough to win two-thirds of the Senate to actually convict him. Unless a party has enormous partisan majorities, a partisan impeachment effort is doomed to fail.