They do well. Generally, retiring their tenure wealthier than their congressional salary would allow at a 100% savings rate.
https://www.jstor.org/stable/10.1017/s0022381613000194
> For all trades reported electronically and on paper, the buys and sells have the same pattern; they underperform slightly for the first few months and then outperform the market after 150 days by less than 1 percent, which is not statistically significant. Otherwise, the returns of the buys and sells appear to be relatively random.
> The results show that neither Democrat nor Republican senators are especially skilled at picking stocks to buy.
https://home.dartmouth.edu/news/2020/04/us-senators-stock-pi...
The holdings and advanced metrics are behind a paywall (apologies in advance).