Additionally, The senate is the only "Court" that can hear impeachment trials, so there is no appeals system to overturn the decision in such a matter (The Supreme court effectively washed their hands of any involvement in Impeachment in Nixon v. United States (no not that Nixon.).
Additionally, in U.S. Politics, the political parties have little in the way of recourse against legislatures who vote against the party line that can be legally done to the member who voted improperly (while I understand Westminster-style Parliamentary ministers can be reprimanded by the Party for voting the wrong way in some cases, I'm not up on the details). If a Senator breaks with the party line, it's under threat of the constituent's refusal to re-elect in the next election cycle and the Senate is not elected whole cloth at any one time (rather one third of the senate is always less than or two years away from their next election). So even discounting blowback by the constituents back home across the board, if all the senators not up for election are voting to Impeach, 2/3rds majority can still happen... but there are members in both parties who are more than two years out.
And voting a senator out because you do not like the way he or she voted in impeachment is perfectly legal as is the senator realizing he won't get elected again specifically based on his/her decision. In fact, this threat was part of the equation specifically to dissuade the majority from impeaching the President for the crime of being a member of the minority party... it had better be a damn good reason for impeaching and not political pettiness.
Also, as a final point out, no vote in Congress taken on the floor may be secret to the voters (jury deliberations are held in secret and the foreman of the jury reads the decision of the vote, which is a secret ballot as far as the court is concerned (there are no rules about knowing who voted which way among the jurors however, and they are permitted to show how close the vote was once dismissed from the case at conclusion. The senate does deliberate the impeachment in secret, but the vote is in the public eye, so you will know who voted for and against).